Suppressor cell activity in two individuals (S1 and S2) with immunodeficiency with thymoma (ID-THY) was studied in peripheral blood mononuclear cells in pokeweed mitogen-stimulated single culture and cocultures. Secreted Ig was measured by radioincorporation and immunoprecipitation after 5-7 days. Control cocultures of normal/normal cells showed, in most cases, a percentage observed to expected ratio (% O/E) of cpm Ig near 100%. However, augmentation (% of O/E > 150) was often encountered, whereas suppression (% O/E < 50) was found only once in forty-two cocultures. In ID-THY/normal cell cocultures the degree of suppression by ID-THY cells varied widely when the same or different cocultivants were retested. This finding could be explained in part by an inverse correlation between the amount of secreted Ig produced by normal cells in single culture and the degree of suppression of the same normal by ID-THY cells in cocultures. A panel of normal cells were all suppressed when a range of S1 or S2/normal cell ratios were tested, weighing against genetic differences in suppressibility in the above system. ID-THY cells failed to block Ig secretion of human lymphoblastoid line cells, suggesting that the mechanism of suppression was related to a block in differentiation rather than interference with Ig synthesis per se. An experiment using cocultivants separated by a Millipore membrane showed that suppression was mediated by a humoral factor.