Inapparent nodule-transformed cells were recovered from five late-pregnant, first-pregnancy BALB/cfC3H females 4 months of age and from five late-pregnant multiparous females 6 to 7 months of age. Mammary tissues were removed from each donor and dissociated by means of the enzyme collagenase (0.1%), hyaluronidase (0.1%), and pronase (1.25%). Aliquots of 100,000 viable cells in 0.01 ml of media were injected into the gland-free mammary fat pads of 3-week-old syngeneic host mice. Ten weeks after the injection the outgrowths were classified as ductal, nodule, tumor, or combinations of these types of outgrowths. The recovery of nodule outgrowths indicated the presence of nodule-transformed cells in the cell suspension that was injected. All donors yielded nodule outgrowths, and the percentage of outgrowths was significantly greater than was the percentage recovered from virgin BALB/cfC3H females of the same age groups. The latent period for the emergence of nodules and tumors was reduced from 8 to 9 months in virgin females to 4 months in parous females. The incidence of both nodules and tumors was greatly increased. The data suggest that parity significantly increases the numbers of nodule-transformed cells in donor tissue, decreases the time required for the emergence of nodules and tumors, and increases the number of overt nodules and tumors.