The amounts of phospholipid‐and Ca²⁺‐dependent protein kinase (PKC) of various human mammary tumor cells containing (ER+) or lacking (ER‐) estrogen receptors were estimated by quantitative immunoblotting. According to several criteria the polyclonal anti‐PKC antibody raised in rabbits against porcine brain PKC specifically recognizes an 80‐kDa polypeptide on immunoblots. This 80‐kDa PKC presumably represents the autophosphorylated form of the holoenzyme. Immunological quantitation of PKC revealed that the levels of immunodetectable PKC varied widely among the various human mammary carcinoma cell lines but closely matched the amounts determined by enzyme activity and phorbol ester binding in the respective cell line. The largest amounts of immunodetectable PKC were found in the ER‐human mammary tumor cells (0.5 to 1.5 μg PKC/mg of cytosolic protein). These data indicate that ER‐human mammary carcinoma cell lines express significantly higher levels of PKC than their estrogen‐receptor‐containing counterparts.