The question whether B lymphocytes are capable of being activated by antigen in the absence of functional T cells was investigated in a model that excludes participation of T cells by virtue of an immune response gene restriction. Strain 2 guinea pigs are capable of responding to immunization with DNP-PLL, whereas strain 13 animals are not. In the present experiments, animals of both strains were immunized with DNP-PLL complexed to ovalbumin (DNP-PLL-Ova) under conditions in which equal titers of antibodies to DNP were produced by both strains. The failure of T cells of strain 13 animals to respond to DNP-PLL was confirmed by the virus plaque assay. While spleen cells from both strains produced MIF after stimulation with DNP-PLL-Ova, in response to DNP-PLL only strain 2 spleens were able to produce MIF. Cells from neither strain could be activated by DNP-guinea pig albumin to produce MIF. We conclude that B lymphocytes are incapable of being stimulated by antigen in the absence of T cells, and that MIF production is a thymus-dependent response. While the results indicate that MIF production is a valid qualitative assay for T-cell competence, since MIF can be produced by B and T cells, the degree of migration inhibition cannot be regarded as a quantitative measure of T-cell function.