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C57BL/6J mice were treated with ALS, ALG, normal rabbit serum (NRS) or IgG of the rabbit for a period of 6 days To study the xenogeneic antigenicity of ALG and its antilymphocytic effect separately, half the mice were made tolerant to IgG of the rabbit. While a short schedule of injections of ultracentrifuged lgG induced tolerance to IgG, this tolerance was eliminated by the subsequent treatment with ALG. A prolonged injection schedule over 44 days did, however, lead to a tolerance which persisted after ALG treatment. [131]IgG elimination proved to be a more sensitive control of tolerance than immunodiffusion. Semiquantitative cytology revealed a decline in lymphocytes, erythroblasts and megakaryocytes in the bone narrow and erythroblastosis and megakaryocytosis in the spleen. Cells of the myeloid series invariably increased in number. The effect on erythroblasts persisted after removal of contaminating haemagglutinins. ALG-treated animals intolerant to IgG had about four times more splenic plasma cells than ALS-treated mice tolerant to IgG. The 6 days' treatment with ALS reduced the, absolute number of nucleated bone marrow cells by 16% and doubled the spleen indices. Under this treatment, relative and absolute cell counts revealed an ALG-induced shift of erythroblasts, megakaryocytes and perhaps also of plasma cells from the bone marrow to the spleen. The effect of NRS or IgG was similar to that of ALS or ALG but remained on a lower level. The remarkable changes in the histologic structure of the splenic follicles after treatment were almost completely reversed by a prior induction of tolerance to IgG. [ABSTRACT FROM AUTHOR]