Five tumor subpopulations have been isolated from a single, strain Balb/cfC3H mouse mammary tumor. These subpopulations have been found to differ in regard to morphology, growth characteristics, karyotype, expression of unique and virus-associated tumor antigens, sensitivity to cytotoxic drugs, and sensitivity to x-irradiation. Four of the subpopulations are stable, whereas the fifth is capable of giving rise to new, tumorigenic variants. The immunopharmacologic consequences of tumor heterogeneity are being studied by observing the growth characteristics, immunogenicity, and drug sensitivity of selected mixtures of the subpopulations growing in vivo and in vitro. So far, the growth of individual subpopulations has been shown to be influenced by the presence of another subpopulation. Immunologic responses seem to be involved in these interactions, as well as non-cellular mediator which is produced by two of the subpopulations and released into culture medium.