Eight cases of primary breast carcinoma were studied for evidence of heterogeneity by flow cytometry using propidium iodide staining of DNA. Tumors ranged from 2.5 to 8.0 cm in greatest dimension, mean 4.2 cm. Five to 11 samples, determined by tumor size, were analyzed from each primary tumor in addition to 4 axillary lymph nodal metastases from 3 of the 8 cases. The presence of carcinoma was demonstrated histologically in each flow cytometric sample. Three of the carcinomas showed only diploid DNA content. Three showed the same single hyperdiploid population in each sample. Two carcinomas (2.5 and 3.5 cm) were heterogeneous and demonstrated 2 or 3 hyperdiploid populations. The 2 or 3 distinct hyperdiploid populations were present singly or sometimes in combination in one sample. Five of the 11 samples from 1 of the heterogeneous tumors and its axillary metastasis contained a single diploid population. Therefore, flow cytometric analysis of a single sample from a breast carcinoma may not be representative of the entire tumor. The study of multiple samples of breast carcinomas by flow cytometry is necessary for full characterization of the tumors.