The Ca²⁺- and phospholipid-dependent protein kinase, protein kinase C (PKC), is a critical enzyme in the regulation of cell growth. In this report, we demonstrate elevated expression of PKC activity in surgical specimens of eight of nine spontaneous human breast tumors, as compared with the expression of PKC activity in normal breast tissue obtained from the same patients. The mean PKC specific activity in histologically normal breast tissue was 166 ± 63 pmol ³²P/min/mg, whereas the mean PKC specific activity in the breast tumors was 460 ± 182 pmol ³²P/min/mg (P = 0.0003; Student's t test). The low interpatient variability among the PKC levels observed in the histologically normal breast tissue specimens and the significant elevation of PKC levels observed in the tumors indicate that elevated expression of PKC activity in breast tissue is a potential marker for malignant disease in the breast.