Passively transferred thymocytes and spleen cells from donors primed with keyhole limpet hemocyanin (KLH) exerted differential suppressive effect on IgM and IgG antibody responses of syngeneic recipients immunized with DNP-KLH depending primarily on the time when KLH-primed cells were transferred. This was demonstrated by the decrease in the numbers of DNP-specific direct and indirect PFC in the spleen of the recipients given KLH-primed cells at different times during primary and secondary immunization. Whereas the cell transfer simultaneously with or 2 days after the primary immunization produced only slight suppression of the peak IgM antibody response, it caused profound suppression of late IgM and IgG antibody responses. By contrast, the cell transfer 3 days after the immunization produced immediate suppression of the ongoing IgM antibody response resulting in its earlier termination, while being unable to prevent the induction of IgG antibody response. KLH-primed cells could moderately suppress the secondary anti-DNP antibody response, in which IgG antibody response was found to be slightly more sensitive than IgM antibody response to the suppressive influence of KLH-primed cells. The suppressive effect of the KLH-primed spleen cells was completely eliminated by the in vitro treatment of the cells with anti-theta and C before cell transfer, indicating that cells responsible for the suppression are, in fact, T cells. The suppression of DNP-specific antibody response by KLH-primed T cells was achieved only if the recipients were immunized with DNP-KLH but not with DNP-heterologous carrier, suggesting that direct interaction between T and B cells is necessary for the suppression of the antibody response. It is concluded that susceptibility of B cells to the specific suppressive influence of T cells is inherently different depending on the differentiation stage of B cells and on the immunoglobulin class they are destined to produce.