A spontaneously transformed T suppressor (Ts) clone, A12-D11/t, is described which arose from the antigen-specific Ts clone A12-D11 isolated from cells of the inguinal lymph node of a BALB/c mouse bearing the syngeneic plasmacytoma ADJ-PC-5. A12-D11/t Ts cells suppress in vitro specifically a primary syngeneic cytotoxic antitumor response with the consequence that no ADJ-PC-5-specific cytotoxic T cells can be generated. The in vivo effects of A12-D11/t Ts cells were studied by injecting them into BALB/c mice. Spleen cells from those mice subsequently failed to respond against ADJ-PC-5 plasmacytoma cells, whereas their response against other syngeneic BALB/c tumors remained unaffected. By using a model system which allows us to study the host's immune reactions to the antigenic load corresponding to initial stages of tumorigenesis, it has been shown previously that ADJ-PC-5-specific Ts cells are activated before the antigen threshold for the activation of cytotoxic T cells is reached. Regarding its phenotype and specificity, the A12-D11/t Ts clone seems to be the exact counterpart of such a Ts cell, and is therefore of special interest in the study of the role of Ts cells preventing immunity against a growing tumor.