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One human skin fibroblast (HSF), three normal mammary epithelial (HNMEC), four mammary carcinoma (HMCC) and three malignant melanoma (HMMC) cell lines were examined for responsiveness to 17 alpha- and 17 beta-estradiol. Responsiveness was monitored by changes in 3H-thymidine uptake (DNA synthesis), 3H-leucine uptake (protein synthesis), extranuclear protease activities and resistance to complement-dependent serum cytotoxicity (CDSC). 17 alpha-estradiol had no effect on any of the examined cell lines. 17 beta-estradiol had no effect on the extranuclear protease activities, or resistance to CDSC, but it increased DNA and protein synthesis in HSF and HNMEC cell lines. 17 beta-estradiol increased sensitivity to CDSC cellular lysis, DNA and protein synthesis. It induced qualitative and quantitative changes in the extranuclear protease activities suggesting an estradiol-dependent enzyme. Extracts of estradiol-treated human mammary carcinoma and human malignant melanoma cells yielded concentrated protease activities. Fractionation of the extracts gave enzymically active protein which differed in their catalytic properties.