Nulliparous, CFW mice were injected with 25 m̈c of tritiated thymidine on day 19 of pregnancy, and days 1, 2, 3, 5, 7, 10, 15 and 20 of lactation. The animals were killed one hour after injection. The inguinal mammary glands were removed and processed for paraffin sectioning. Radioautographs were prepared, using the dipping technique.

Quantitation of mammary epithelial cell proliferation for the intra- and interlobular (ducts) epithelium was performed by determining the percent of labeled epithelial cells in a large sample of cells (labeling index). It was concluded that epithelial cell sample sizes of 1,000–2,000 cells were adequate to measure mammary epithelial proliferation. A wave of epithelial proliferation was observed during early lactation. In the intralobular epithelium, a peak labeling index of 11.1% was attained on day two of lactation whereas a peak labeling index of 7.9% was observed on day three of lactation in the interlobular epithelium. Cells of the connective tissue and vascular bed proliferated in response to the growth of the mammary epithelium. Myoepithelial cells were frequently labeled on days two and three of lactation.