It has "been shown for the first time that isoimmunisation of sows by foetal red cell antigens can occur. By studies on two sows in a minimal disease pig unit, it was demonstrated that iso-antibodies were produced to the blood group factor, Ea, after giving birth to Ea positive piglets but not after giving birth to Ea negative piglets alone. Complete and reliable life histories of both sows were available and the possibility of previous injections of pig red cells in any form was precluded. Further studies were carried out on two Ea negative, Kb negative sows which were mated to an Sa positive, Kb positive boar. In one of these, anti Ea was boosted after farrowing as in the previous two cases, while in the other the anti-Ea titre rose about six weeks before parturition. In addition anti-Kb was produced in both at the same time as anti-Ea. A subsequent mating of the first of these to an Ea negative, Kb positive boar resulted in the boosting of the anti-Kb titre but not the anti-Ea titre, again after parturition. Likewise on mating the second sow to an Ea negative, Kb negative boar, there was little effect on the anti-Ea and anti-Kb titres either throughout pregnancy or after parturition. Two Ea negative gilts and a further Ea negative sow, all of which had no evidence of red cell iso-antibodies in their sera were mated to an Ea positive boar. Anti-Sa was detected in the sera of two of them for the first time after the subsequent parturition; the third remained negative. On examining the sera from eighty sows and sixty-seven boars in the routine typing service carried out by the Blood Group Research Unit, no antibodies other than anti-A were found in the boars' sera while, in twenty-six per cent of the sows' sera, red cell iso-antibodies were found as well as anti-A. All these sows were bred to boars of the same breed as themselves. Similarly, the sera of fifty per cent of twenty-four sows which had produced litters affected with thrombocytopenic purpura and which were mainly bred to boars of different breeds than themselves, had iso-antibodies present Of these iso-antibodies, anti-Ea and anti-Eb were the most prevalent and also present at the highest titres. Anti-Ee, anti-Pa, anti-Ka, anti-Kb, anti-Kd, anti-La and anti-Lg were also found but usually at only low titres. The effect of anti-Ea on the red cells of piglets in four litters was studied. Although the antibody was absorbed from the colostrum by all the piglets, it had little effect on their haematological pictures and no differences were noted between Ea positive and Ea negative piglets in the same litter. All these results support further the hypothesis that iso-immunisation of sows by incompatible foetal antigens does occur. In support of this hypothesis a condition of piglets known as thrombocytopenic purpura has been reported in several Scandinavian countries as well as in the United Kingdom. It is considered that iso-immunisation of the sow by thrombocytes of the foetuses takes place in vitro or at parturition resulting in the destruction of thrombocytes in the neonatal piglets after absorption of antibodies from their dam's colostrum. Four litters affected "by this condition were studied in detail and clinical symptoms similar to that described by the other workers were noted. Two gilts, non-affected members of a litter which suffered from the condition, were mated to a boar, an affected survivor of a similar litter. During pregnancy the gilts were injected with pure thrombocyte suspensions from the boar. The piglets produced from these matings had normal thrombocyte counts at birth but a proportion of them developed thrombocytopenia and purpura similar to that seen in natural cases within a few hours of receiving colostrum. However, despite the fact that this condition was observed in three litters from each of these gilts, a secondary thrombocytopenia at ten to fourteen days of age, characteristic of the naturally occurring condition, was only observed in two piglets in one litter. Of all techniques investigated the antiglobulin consumption test was found to be the most satisfactory for the detection of thrombocyte antigen/iso-antibody reactions. Using this, the thrombocytes of affected and non-affected piglets in the litters experimentally affected with thrombocytopenic purpura, were tested against their dam's serum. A correlation was demonstrated between the serological reactions of the piglets' thrombocytes and the clinical signs of purpura and thrombocytopenia.