Previous studies have identified protein kinase C as a potent regulator of SP-A expression in human lung cells (H-441) since treatment of H-441 cells with a PKC-activating phorbol ester (TPA) inhibited SP-A mRNA expression (J Biol Chem 255:20822, 1990). In the present study, we identified the isozyme of PKC responsible for regulation of SP-A mRNA expression. Using Western blot analysis, we first identified isozymes of PKC expressed by H-441 cells. These cells express PKC α, η and ζ, but did not express β,γ, δ or ε. Of the expressed isoforms, only α and η are phorbol ester responsive. Next, H-441 cells were treated with TPA (100 nM), and PKC isozyme expression determined by Western analysis. TPA treatment caused a dose- and time-dependent decrease in PKC α protein, but did not alter PCK η. The time-dependent decrease in PKC α protein levels was accompanied by a time-dependent decrease in SP-A mRNA expression. These data support the hypothesis that PKC α regulates SP-A mRNA expression in H-441 cells since disappearance of PKC α protein is accompanied by decreased SP-A mRNA expression. The mechanism by which PKC α exerts this effect is presently being investigated. Supported by HL78764 (WRR) and NIH Training Grant HL07527 (MJL).