Two kinds of phosphoinositide-specific phospholipase C (PLC) were purified from rat liver by acid precipitation and several steps of column chromatography. About 50% of the activity could be precipitated when the pH of the liver homogenate was lowered to pH 4.7. The redissolved precipitate yielded two peaks, PLC I and PLC II, in an Affi-gel Blue column, and each was further purified to homogeneity by three sequential h.p.l.c. steps, which were different for the two enzymes. The purified PLC I and PLC II had estimated Mr values of 140,000 and 71,000 respectively on SDS/polyacrylamide-gel electrophoresis. Both enzymes hydrolysed phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) in a Ca2+- and pH-dependent manner. PLC I was most active at 10 microM- and 0.1 mM-Ca2+ for hydrolysis of PI and PIP2 respectively, whereas PLC II showed the highest activity at 5 mM- and 10 microM-Ca2+ for that of PI and PIP2 respectively. The optimal pH of the two enzymes also differed with substrates or Ca2+ concentration, in the range pH 5.0-6.0. Hydrolysis of phosphoinositides by these enzymes was completely inhibited by Hg2+ and was affected by other bivalent cations. From data obtained by peptide mapping and partial amino acid sequencing, it was clarified that PLC I and PLC II had distinct structures. Moreover, partial amino acid sequences of three proteolytic fragments of PLC I completely coincided with those of PLC-148 [Stahl, Ferenz, Kelleher, Kriz & Knopf (1988) Nature (London) 332, 269-272].