The growth of B-cell lymphoma, LSCC-RP9, in culture was inhibited by spleen cells from bursa-immunized agammaglobulinemic (A-gamma) chickens. This inhibition was mediated by suppressor T-cells. The growth of transplantable LSCT-RP6 B-cell lymphoma was suppressed in A-gamma chickens, while that of the control SPCT-RP11 T-cell tumor was not affected. Furthermore, the incidence and growth of the LSCT-RP6 tumor in normal recipients were decreased when it was co-transplanted with spleen cells from bursa immunized A-gamma chickens. The results suggest that suppressor T-cells inhibit the growth of B-cell lymphoma.