The proenzyme of a Ca2+-dependent protease-activated protein kinase previously obtained from mammalian tissues (Inoue, M., Kishimoto, A., Takai, Y., and Nishizuka, Y. (1977) J. Biol. Chem. 252, 7610-7616) was enzymatically fully active without limited proteolysis when Ca2+ and a membrane-associated factor were simultaneously present in the reaction mixture. The activation process was reversed by removing Ca2+ with ethylene glycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid. An apparent Ka value for Ca2+ was less than 5 x 10(-5) M. Other divalent cations were inactive except for Sr2+, which was 5% as active as Ca2+. The factor was almost exclusively localized in membrane fractions of various tissues including brain, liver, kidney, skeletal muscle, blood cells, and adipose tissue. It was easily extractable with chloroform/methanol (2:1), and was recovered in the phospholipid fraction. In fact, this membrane factor could be replaced by chromatographically pure phosphatidylinositol, phosphatidylserine, phosphatidic acid, or diphosphatidylglycerol. Phosphatidylethanolamine, phosphatidylcholine, and sphingomyelin were far less effective under the comparable conditions. Ca2+-dependent modulator protein was unable to support enzymatic activity. The enzyme thus activated showed an ability to phosphorylate five histone fractions and muscle phosphorylase kinase, and appeared to possess multifunctional catalytic activities.