The time course of appearance of hyperplastic alveolar nodule(s) (HAN) and mammary tumor(s) (MT) was determined in untreated virgin "viable yellow" (Avy/A) and non-yellow (A/a) (C3H/HeNIcrWf X VY/Wf)F1 female mice. The first HAN was detected in a yellow female at age 16 weeks, the first period in which mice were killed. HAN were not found in the non-yellow mice until they were 19 weeks old. The incidence of HAN increased to 92% among yellow females and to 75% among non-yellow females by 36 weeks of age. MT were first observed at age 22 weeks in yellow mice and at 28 weeks in non-yellow mice. The incidence of MT at 36 weeks was 75% among yellow mice and 22% among non-yellow mice. Type B adenocarcinoma was the predominant class of MT found in the yellow mice. The time courses of appearance and incidence of HAN and MT in the non-yellow F1 mice were similar to those observed in inbred C3H female mice. MT first appeared in each population when the incidence of HAN bearers had reached 40--45% regardless of age, body weight, or number of HAN per HAN bearer. Apparently, the phenotypic effects of the Avy gene primarily stimulated the multiplication of nodule-transformed cells to form HAN and thus indirectly enhanced MT formation.