Females of the fostered C3H strain and the BAF(1) hybrid generation, having a normal incidence of breast tumors of 1-2 per cent., were given, by mouth or injection, filtrates (Seitz filter) or extracts of glycerinated-treated tissue containing the active milk-influence for the development of spontaneous breast cancer. Sixty-three experimental mice were observed to have an incidence of 41 per cent. Thirty-six mice received unfiltered or untreated material and have had an incidence of 67 per cent. Many of the mice of each group are still living. Previous studies indicated that the active milk-influence would not become inactive following desiccation.(4) Following ultracentrifugation the active influence appeared in traces, if at all, in the fat fraction and in the final supernatant fluid, and it is possible that the active agent is a colloid of high molecular weight.(5).