When rat protein kinase C-delta (PKC-delta) was produced by Sf9 cells infected with a recombinant baculovirus in shake-flask culture using a serum-containing medium, the intracellular PKC-delta content decreased in the late period while the extracellular PKC-delta markedly increased. During the late period of serum-free culture, the extracellular PKC-delta level considerably declined, but the addition of a protease inhibitor, leupeptin, prevented the reduction in PKC-delta production.