Protein kinase C (PKC) has been implicated in the control of airway smooth muscle (ASM) tone, and abnormalities in PKC-dependent signaling may be associated with asthma. PKC exists in different isoforms, but the pattern of their expression in ASM has not been previously reported. Accordingly, the purpose of the present study was to identify which isoforms of PKC are expressed in ASM. Tissue samples of canine ASM were homogenized and subjected to sodium dodecyl sulfate polyacrylamide gel electrophoresis, followed by immunoblotting with a range of antipeptide antibodies to PKC-alpha, -beta I, -beta II, -gamma, -delta, -epsilon, -eta, -theta, and -zeta. Positive controls were run in parallel with ASM. Immunoblots revealed a mixture of both calcium-dependent and calcium-independent isozymes: PKC-beta I and PKC-beta II were the only conventional isoforms detected; PKC-delta, PKC-epsilon, and the new muscle-specific isoform, PKC-theta, were all expressed in ASM, but the lung-specific isoform, PKC-eta, was not detected. The calcium- and phospholipid-independent isoform, PKC-zeta, was also present. Thus, expression of a wide variety of both calcium-dependent and calcium-independent isoforms suggests a complex, multifunctional role of PKC in ASM.