Src-family kinase expression was measured in 52 human mammary tumor (T) specimens compared with non-tumor (NT) tissue from the same patient by enzymatic assays employing a Src-kinase family-specific peptide substrate and by immunoblotting with an antibody recognizing the Src-family kinases Src, Fyn, and Yes. In the T specimens, the mean enzymatic activity was moderately elevated (T: 160 fmol ATP min-1 mg-1; NT: 115 fmol ATP min-1 mg-1) with 25 tumor samples having higher activity than the corresponding NT tissue, 17 having lower activity, and no activity detectable in ten T/NT pairs. Immunoblotting revealed clearly elevated expression in 25 tumor tissues and no differences or expression below the detection limit in the remaining T/NT pairs. The data are in agreement with a possible role of Src-family kinases for the biology of mammary carcinoma.