Amplification of oncogenes in mammary carcinoma shown by fine-needle biopsy. A procedure that measures the amplification of oncogenes in human cancer cells is described. The cells were obtained by fine-needle biopsy to allow repeated sampling from individual metastases. A drawback was the low number of cells obtained, but this could be overcome by using a slot-blot hybridization technique to measure gene amplification. Two patients with mammary cancer (primary tumors or metastases), analyzed for the levels of amplification of the oncogene erb-B2, are described in detail. This technique is suitable for analyzing alterations occurring during cancer progression and for identifying subgroups of mammary cancer with different characteristics. 