Objective: The aim of this study was to compare the distributions of protein kinase C isozymes in human nonpregnant and pregnant myometrial tissues and primary cell cultures. Study Design: Myometrial tissues were obtained at hysterectomy from nonpregnant women and at cesarean delivery from women both before and during early labor at term. Western immunoblot analysis was performed on homogenates of myometrial tissues and primary cell cultures with monoclonal antibodies specific for protein kinase C isozymes. Redistribution and translocation of protein kinase C were examined by means of immunocytochemical methods. Results: Nonpregnant myometrial tissues contained protein kinase C isozymes ⍺, γ, δ, μ, ι, and ζ but not β₁, β₂, θ, or ϵ. Pregnant myometrial tissues both before and during early labor contained the same protein kinase C isozymes and also β₁ and β₂. The protein kinase C isozyme distribution in primary myometrial cell cultures was identical to that in the myometrial tissues. Protein kinase C redistribution and translocation were demonstrated in these cultured myometrial cells. Conclusion: Both human myometrial tissues and primary cell cultures expressed a broad range of protein kinase C isozymes. Protein kinase C isozymes β₁ and β₂ were absent in nonpregnant myometrium but were induced during pregnancy. Labor at term did not alter protein kinase C isozyme expression. (Am J Obstet Gynecol 2000;183:1525-31.)