A quantitative evaluation of IgA, IgD, IgG and IgM plasmocytes in sequential bone marrow aspirates of patients with acute leukemia using the peroxidase-antiperoxidase method was undertaken. Plasmocytosis resembled that of normal controls or was slightly subnormal on admission. When remission was obtained, bone marrow plasmocytosis was similar to normal controls, irrespective of the type of acute leukemia and the cytostatic treatment. Occurrence of infection strongly augmented the number of plasmocytes, with an initial increase in IgM and later in IgG plasmocytes. This suggests that the immune response is preserved in patients with acute leukemia.