In a previous study, we purified PAF-acetylhydrolase, which converts PAF to an inactive metabolite, lysoPAF, from peritoneal fluid of guinea pigs subjected to experimental endotoxin shock and found that this purified enzyme had similar biochemical properties to the plasma enzyme [Karasawa, K., Yato, M., Setaka, M., and Nojima, S. (1994) J. Biochem. 116, 374-379]. In this study, we isolated a homogeneous enzyme preparation from guinea pig plasma using a similar procedure. The molecular mass of this purified enzyme, as determined by SDS-PAGE was 58-63 kDa, larger than that (43 kDa) of the human enzyme. To elucidate the molecular structure of this enzyme and clarify its relationships with PAF-acetylhydrolases of other species, we isolated and sequenced a cDNA encoding this enzyme. Its cDNA contains an open reading frame encoding 436 amino acids and its predicted molecular mass (49 kDa) is lower than that of the native enzyme, suggesting that guinea pig plasma PAF-acetylhydrolase, unlike the human enzyme, is modified post-translationally, perhaps by glycosylation.