Five subpopulations (66, 67, 68H, 168, and 4.10LM) obtained from a single BALB/cfC3H mammary adenocarcinoma were used to assess intratumor immunologic heterogeneity. BALB/c and BALB/cfC3H mice were immunized with each of the subpopulations, and lymph node cells (LNC) from immunized animals were tested for cell-mediated immunity (CMI) to each subpopulation in vitro by chromium release and microcytotoxicity tests and in vivo by Winn assays. The immunogenic character of the subpopulations differed markedly. The pattern of cross-reactivity indicated that at least two determinants were involved, one of which was probably a viral antigen. The viral antigen was expressed on 4 subpopulations (66, 68H, 168, and 4.10LM). The other determinant was immunogenic in both BALB/c and BALB/cfC3H mice and was expressed on 4 subpopulations (66, 67, 168, and 4.10lm). Thus 1 subpopulation (68H) expressed only the viral antigen, 1 (67) expressed only the other antigen, and 3 (66, 168, and 4.10LM) expressed both. Expression of the determinants showed qualitative and quantitative variations. Quantitative differences were noted by the relative effectiveness of the subpopulations to induce CMI and by the relative sensitivities to LNC-mediated killing. Qualitative differences were indicated by the occurrence of unidirectional cross-reactivities between some pairs of subpopulations; a determinant could be expressed so that the subpopulation could induce cytotoxic cells but not be sensitive to them or vice versa.