A purified protein kinase C (PKC) has been isolated from term human placental tissue, which is phospholipid and Ca(2+)-dependent. Two subtypes of the enzyme were identified by hydroxyapatite column chromatography and using monoclonal antibodies with immunohistochemical techniques; subtype III is present in higher concentration than subtype II. Their ratio of 2.5 is very similar in second and third trimester placentas, but is higher, 6.5, in the first trimester. The subtype I was never expressed.