Helper T cells were obtained by injecting heavily irradiated semiallogeneic mice with lymph node cells from H-2-incompatible parental strain mice primed with sheep erythrocytes (SRC) 2 mo before. Thoracic duct lymphocytes collected from the recipents 18-40 h later (nearly all of which were theta-positive and of donor origin) were totally and specifically unresponsive against host-type determinants in mixed-lymphocyte culture. The filtered cells were transferred to irradiated semiallogeneic mice together with SRC and anti-theta-serum-treated (B) cells from SRC-primed syngeneic, semiallogeneic, or allogeneic mice. When antibody-forming cells were measured in the spleen 5-9 days later, effective IgM and IgG collaborative responses were observed with both syngeneic and semiallogeneic B cells but not with allogeneic B cells. No evidence was found that the failure to obtain collaboration with the allogeneic B cells was due to inhibition of the B cells by the T cells or vice versa.