Explants of human breast (obtained aseptically from ten women ages 18–37) were examined and the vitamin A compound B-retinyl acetate was tested (at 3 × 10−5 and 3 × 10−6 M) for its effects on the fine structure and growth of the epithelium. In the absence of B-retinyl acetate, cells growing out from the explant (outgrowth cells) underwent squamous metaplasia, began to accumulate many intermediate filaments (tonofilaments), and revealed large desmosomes after 2 weeks in culture. In the presence of either concentration of B-retinyl acetate, the epithelial cells were largely prevented from undergoing squamous metaplasia. The glandular epithelium inside the explant maintained a typical secretory appearance for 2 weeks in control cultures (without the retinoid) and then began to show increased numbers of lysosomes and a loss of secretory granules. These glandular epithelial cells did not undergo squamous metaplasia, but they contained increased numbers of intermediate filaments. In contrast, glandular epithelium cultured in either concentrations of B-retinyl acetate appeared secretory for as long 6 weeks in culture, and intermediate filaments were not obvious. Autoradiographs demonstrated that both concentrations of B-retinyl acetate inhibited cell division in the outgrowth epithelium and in the (internal) glandular epithelium. The ability of B-retinyl acetate to reverse squamous metaplasia in the outgrowth cells was also tested. Out growth cells reversed to a more normal ductlike appearance after 6 weeks culture in standard medium followed by only 1 week in 20 μg/ml B-retinyl acetate. After 7 weeks in standard medium and 1 week in 10 μg/ml B-retinyl acetate, the cells showed a partial reversal of the squamous metaplasia.