The feasibility of techniques developed for isolating and culturing human mammary epithelial cells of malignant origin was confirmed in 136 primary breast cancers, 116 hypodermal metastases, and 8 metastatic lymph nodes. In 115 (84%) primary breast cancers and in 81 (70%) hypodermal recurrences we observed a good in vitro cellular proliferation. These proliferating cells, at the second passage, were used for a clonal assay suitable for quantitating drug sensitivity. With this clonal assay median cloning efficiencies of 14% and 6% were obtained respectively in primaries and in skin recurrences. We examined the in vitro response to different drugs and confirmed the test's ability to detect heterogeneity in response to same drugs (doxorubicin, 4'-epidoxorubicin, vinblastine, cis platinum, and idarubicinol) among the different breast carcinoma cultures as well as heterogeneity among subpopulations within a single carcinoma.