Epidermal cells were separated from newborn mouse skin and grown on glass chamber slides or in plastic Petri dishes. Cell proliferation was examined at short intervals during the first 3 to 4 days in culture. DNA synthesis was estimated by the incorporation of [3H]thymidine into epidermal DNA, or by the labeling index. The mitotic rate was estimated by blocking mitotic cells with vinblastine. Reasonable agreement was found with the different methods. The cells grew synchronously with peaks of DNA synthesis at 21 to 23 hr, 35 to 40 hr, and 60 hr; peak mitotic rates were seen at 44 to 48 hr and 72 to 76 hr. The cells appeared to grow exponentially during the first 3 to 4 days in culture, with one main cohort of cells dividing during the successive proliferative peaks.