The present study describes the production of human procollagen I in a baculovirus expression system. Recombinant baculovirus carrying pro alpha1(I) or pro alpha2(I) cDNA was constructed and infected to Sf9 cells. Full-length pro alpha1(I) or pro alpha2(I) chains were synthesized by the cells infected with either of the recombinant viruses. The pro alpha1(I) chains formed pepsin-resistant homotrimers stabilized by interchain disulfide bonds, a small proportion of which was secreted into the culture medium. The pro alpha2(I) chains were not linked into trimers by disulfide bonds and failed to form stable triple helices, although some chains were suggested to exist as dimers or unstable trimers in which only two chains were linked by disulfide bonds. In spite of their non-helicity, the pro alpha2(I) chains were secreted at a higher rate than the pro alpha1(I) chains. Sf9 cells simultaneously synthesized both pro alpha1(I) and pro alpha2(I) chains when the cells were co-infected with the two recombinant viruses. Pepsin-treatment of the product clearly demonstrated the production of procollagen I heterotrimers composed of two pro alpha1(I) chains and one pro alpha2(I) chain, homotrimers of the pro alpha1(I) chains being negligible. This expression system appears to offer a unique means of studying the mechanism of chain association and secretion during procollagen biosynthesis.