Mice of strain BALB/c, taken from their mothers within 24 hours after birth, were suckled by foster mothers of high-tumor strains C3H, A, DBA/1, and RIII. Controls were BALB/c mice nursed by their natural mothers. The 4 groups of foster-nursed mice, CfC3H, CfA, CfDBA, and CfRIII, differed only in the source of the mammary-tumor agent ingested with the milk; the unfostered mice were presumably agent-free. All mice were force-bred, and examined weekly for mammary tumors; all tumors were diagnosed and typed histologically. Incidences of mammary tumors were high in all fostered groups: 90.9 percent in 55 CfC3H, 88.5 percent in 61 CfA, 88.2 percent in 51 CfDBA, and 79.2 percent in 53 CfRIII; incidence was low (5.1%) in 78 control mice. Although incidences among the fostered groups were not significantly different, average tumor ages were. The average tumor age of the CfC3H group was 7.8 months; the mean ages in the other three groups were 9.3 months in CfA, 9.6 months in CfDBA, and 10.7 months in CfRIII. The agent in the milk of strain C3H mice induced tumors more rapidly in strain BALB/c mice than the agent in the milk of strains A, DBA/1, and RIII. There were indications, but not clear-cut evidence, that the agent in the milk of strain RIII mothers was less potent than that in strains A and DBA/1. These data are evidence that the mammary-tumor agent exists in forms which differ in their virulence or tumor-inducing capacities.