The level of phosphorylation of any cellular protein depends on the balance of the activities of protein kinases and protein phosphatases that act on the protein. In this study, we have characterized, in intact human blood platelets, the activity of protein phosphatase (s) that reverse the action of protein kinase C (PKC), using as a substrate, endogenous 42 kDa protein which has been previously phosphorylated by PKC. In this study 1,2-dihexanoyl-sn-glycerol (DHG) was used to stimulate PKC, diacylglycerol kinase inhibitor-R59022 was used to maintain the activity of PKC and staurosporine and okadaic acid were used to inhibit PKC and protein phosphatases respectively. Our observations indicate that: (1) protein phosphatase 1 (PP1) and/or protein phosphatase 2A (PP2A) are likely to be the enzymes that reverse the phosphorylation activity of PKC on the 42 kDa protein; (2) PP1 and/or PP2A dephosphorylate sites which have been previously phosphorylated by PKC; and (3) PP1 and/or PP2A dephosphorylate, on the 42 kDa protein, both serine and threonine residues, which have been previously phosphorylated by PKC.