Evaluation of in vitro bromodeoxyuridine labeling of breast carcinomas with the use of a commercial kit. The prognostic significance of S-phase fraction analyses of breast carcinomas has been reported by several investigators. The Cell Proliferation Kit (Amersham Corporation, Arlington Heights, IL), which uses in vitro bromodeoxyuridine (BRDU) labeling to evaluate cell cycle kinetics without a flow cytometer or radioisotopes, simplifies this assay for the clinical-based laboratory by providing standardized reagents and recommended methods. This study was performed to determine whether data derived from its use were comparable to published S-phase data from the use of thymidine labeling, BRDU, or other methods on breast carcinomas. Primary infiltrating ductal carcinomas (n = 142) and primary lobular carcinomas (n = 6) showed mean and median BRDU labeling of 4.63% and 3%, 1.3% and 1%, respectively, with a range of 0-28%. Benign lesions always had less than 3% BRDU uptake (n = 21). Estrogen receptor (ER) and progesterone receptor (PR) status correlated with BRDU labeling (P less than 0.05), with the highest S-phase fractions in ER- and PR-negative tumors. Correlations between BRDU uptake and histologic tumor type or size were observed. Significant correlations between BRDU uptake and lymph node status, patient age, or histologic tumor grade were not observed. S-phase studies of breast carcinomas using other techniques have shown similar data, therefore, the Cell Proliferation Kit appears to be a practical and useful method for in vitro S-phase analysis that allows concomitant histologic examination of the same tumor tissue sample. 