A human tumor cloning system has been utilized to grow human breast carcinoma. A total of 225 specimens have been placed in culture. One hundred thirty-two were from primary chest cancer specimens and 93 were from metastatic lesions. Of these, 71% of the primary breast carcinomas and 75% of metastases formed greater than or equal to 5 colonies per 500,000 cells plated. Forty-five percent of the primary breast carcinomas and 52% of the metastases formed enough colonies (greater than or equal to 30 colonies per 500,000 cells plated) to perform meaningful in vitro drug testing. Estrogen receptor status did not influence the percentage of tumors which formed colonies in vitro. Histologic and nude mouse studies provided confirmatory evidence the colonies were composed of breast cancer cells. In 176 in vitro chemotherapeutic drug tests, the anticancer agents commonly used clinically for treatment of breast cancer, i.e., Adriamycin, 5-fluorouracil, etc., showed some in vitro activity. This activity was not as dramatic as is seen in the clinic with these conventional agents. Future work should concentrate on improving the number of colonies which form from breast cancer specimens and on prospective use of the system for screening for new agents for the treatment of human breast cancer.