Protein kinase C (PKC), a family of related but distinct enzymes whose cellular functions are poorly understood, acts in synergy with Ca²⁺ mobilization for the activation of platelets. Using specific antibodies for the different isoforms, immunoblot analysis revealed the presence in human platelets of three different PKC subtypes which specifically react with α, β and ζ-PKC antibodies. Whereas the subcellular distribution of the α PKC remained unaffected, incubation of platelets with 1 μM PMA for 2 min resulted in a significant subcellular distribution from cytosol to membrane of β-PKC (25%) and ζ (15%). The β-PKC isoform is more sensitive than α and ζ-PKC to PMA, since 100 nM PMA resulted in a translocation of 85%, 64% and 66% respectively of a maximum translocation observed with 1 μM PMA.