The work reported in this thesis has shown that the physical state of the antigen has a marked effect on the lysosomal enzyme levels of the spleen. CBA mice were injected with two different physical forms of bovine serum albumin (BSA). One form was of a particulate nature whilst the other was in the form of an aggregate - free material. The particulate form of the antigen was found to be more immunogenic than the soluble, aggregate -free material, which was found capable of tolerance induction. Upon administration of these two antigenic forms, there followed an increase in the acid phosphatase levels of the spleen. The testing of either 106 spleen cells or the large granular fraction, which has been shown to contain mitochondria, microsomes, cell membrane fragments as well as lysosomes, of the spleen yielded similar results. Both test systems when compared with the control animals resulted in an increased acid phosphatase enzyme level. In each case the mixtures assayed represented the acid phosphatase levels of disrupted lysosomes. Intact lysosomes, on the other hand, revealed that the soluble, aggregate -free form of the antigen (C.BSA) caused a similar increase in the amount of acid phosphatase detected as compared to the disrupted organelles after antigenic stimulation. The particulate, heat - denatured BSA, in contrast, gave a marked depression in the amount of acid phosphatase present, as compared to the controls. Similarly, the injection of haemocyanin, which is also immunogenic; as is the particulate BSA, resulted in a depression in the amount of acid phos- phatase levels forty-eight hours after antigenic stimulation. The lymphocyte appears to be one of the cell types which is implicated in these enzyme changes although the role of other cells has still to be established. Column separation of splenic lymphocytes as well as histochemical staining of "purified" lymphocytes suspensions revealed increase acid phosphatase levels in the cells after antigenic stimulation. Autoradiographic studies suggest that the aggregate-free form of the antigen is not localized to the same extent as is the particulate form of BSA. The determination of the total quantity of the respective isotope -labelled antigen, present in the organs examined, reveals that there is no significant difference in the amount of antigen present whether the material be in the soluble or particulate form. The enzyme changes observed must therefore be due to the physical state of the antigen and its effect on the cell types involved and not to the quantity of antigen present at the time of the enzyme determinations. It is postulated that since both forms of the antigen are in equal proportions, as indicated by the counts /mg tissue, in the organs examined, the changes in the enzyme levels observed may be a reflection of mechanisms of the immune process and the induction of tolerance after the administration of various physical forms of the antigen. The involvement of the lysosomes in the induction of tolerance is discussed; the immunogenic material and its effect on the lysosomal enzymes is also discussed.