The frequency of cytotoxic T lymphocyte precursors (CTL-P) reactive to plasmacytoma (PCT) tumour-associated antigens (TAA) was studied in BALB/c mouse spleen. Progressively decreasing numbers of responder BALB/c spleen cells were cultured with mitomycin C-treated stimulator MPC-11 BALB/c PCT cells, and a Poisson analysis was performed of the wells scored positive in a 51Cr release assay with PCT target cells. It was found that approximately 2 per million BALB/c spleen cells were CTL-P reactive to TAA of MPC-11, and about one in three of these precursors responded to a TAA expressed by MPC-11 which was shared with the C3H PCT C1.18. The frequency of CTL-P reactive to TAA was shown to be 30 to 500 fold lower than that for CTL reactive to various major histocompatibility complex alloantigens, and the addition of exogenous Interleukin-2 only resulted in a 2-3 fold increase in this CTL-P frequency. These results suggest that the weak levels of tumour immunity induced in vivo by immunisation of BALB/c mice with MPC-11 are, at least in part, due to a very low frequency of CTL-P reactive to TAA of that PCT.