The activity of macrophages and lymphocytes appears to be abnormal in RZB/B1 mice. Depressed T- cell dependent activity has been noted both here and elsewhere. However, whilst Graft - versus - Host reactions are reported elsewhere to be depressed also, cell transfer experiments within the strain or to histocompatible strains lead to splenomegally in the recipients. Evidence is given which suggests that either a type of Graft - versus - Host or Host - versus - Graft reaction occurs. The abnormal behaviour of the macrophages in this strain reported here may play an important part in the autoimmunity and the abnormal immune capacity of NZB/B1 mice. Phagocytosis was observed to increase as the mice aged, but was accompanied by a decrease in the degradative ability of these P.E macrophages. Hyperactivity in phagocytosis and hypoactivity in the degradation of antigen were obvious when these activities were compared with those of non autoimmune strains of mice. The possible causes of this aberrant behaviour are appraised as is an aetiology of a genetic or viral origin.