Abstract Normal rats and mice showed a significant increase in trinitrophenyl-specific plaque-forming cells following the administration of immunocompetent allogeneic spleen cells. The “response” was maximal at 4–5 days and decreased to background levels by 7–12 days. Induction of a graft-versus-host reaction produced a greater increment in direct trinitrophenyl plaque-forming cells than was found with a host-versus-graft, although the kinetics were identical in both cases. The increase in the number of plaque-forming cells was derived from the host in the graft-versus-host and at least in part from the donor in the host-versus-graft. This effect was not restricted to rats, as it was demonstrable in allogeneic combinations in normal mice and with a xenogeneic (rat → mouse) interaction. The trinitrophenyl specificity detected herein was attributed to prior sensitization with crossreacting environmental antigens. The results suggest that foreign lymphoid cells are able to trigger these cells to differentiate and proliferate into plaque-forming cells.