Lymphoid cells stimulated by soluble tumour antigens in the MCA-induced murine fibrosarcoma system have been identified by subclass and protective capacity in adoptive syngeneic hosts. Lymph-node or spleen cells taken at weekly intervals after inoculation of syngeneic chemically induced fibrosarcomas were enriched by 3 methods in T, B, and "null" cell subclasses, and assayed for proliferative kinetics in response to soluble membrane antigens. The stimulated subpopulations were found to be heterogeneous, their composition varying with time and tumour burden. Initial proliferative responses after tumour inoculation were limited to the T-enriched subpopulation. Later during tumour growth, T, B and null cell fractions were vigorously and equally stimulated by tumour antigen. The ability of the same T, B or null-cell subpopulations to inhibit tumour growth was measured in adoptive hosts by a modified Winn assay. Only the T-cell subpopulation responding to tumour antigen in vitro effectively and consistently retarded tumour growth in vivo. In contrast to the shared specificities on syngeneic tumours identified by the proliferative assay, tumour-growth inhibition was limited to the specific tumour borne by the cell donor.