The distribution of the mouse mammary tumor agent and its antigenicity in milk

Inbred mice with a propensity for mammary tumor development contain the mammary tumor agent and a specific soluble antigen in their milk. These were found in approximately equal proportions in both the supernatant and the pellet after ultracentrifugation. Over 90% of the agent and the highest antigen titer were located in a density range of 1.06–1.075 in ficoll density gradients. The low tumorigenic potential and the antigenicity of the B particle band (p = 1.12) remained with the particles after removal of proteinaceous contaminant by gel filtration. The immunogen from the upper portion (p = 1.06–1.075) of ficoll gradients appears to aggregate when concentrated and eluted from gel filtration columns. The tumorigenic material isolated from ECTEOLA and carboxymethylcellulose chromatographic columns does not show a high positive correlation with the distribution of the virus‐like B particles, which was consistent with the finding that the agent was concentrated in a narrow region with an average density of 1.28 in cesium chloride density gradients. The antigen associated with B particles and the lower molecular weight active component are immunologically identical.