Evidence is presented for gene control over the propagation of the mammary-tumor agent in mice. The segregation data do not support an interpretation of single-gene control, but they suggest that the agent can be propagated in the presence of one or more of several genes. It appears that females lacking any of the genes for propagating the agent do not eliminate it through production of antibodies against it, but apparently the agent is lost in subsequent generations through dilution. There is evidence that females that can propagate the agent vary in the quantity or quality of agent produced, presumably owing to the number they possess of the several genes for the propagation of the agent. The mammary tumors observed in the study are classified as to histologic type, and the other neoplasms of many kinds that were observed are listed.