In vitro cleavage by p15 virion protein of the primary translation product of the avian oncovirus gag gene, the precursor polypeptide Pr76gag, was studied in kinetic experiments. Nondenatured 35S-methionine-labelled Pr76gag, which was synthetized in reticulocyte lysate programmed by genomic AMV-RNA was used as a substrate, pure native p15 (AMV) as a protease. Reaction conditions were optimal for in vitro protein synthesis. Composition of the cleavage products was estimated by immune precipitation with monospecific antisera against internal structural virion (AMV) proteins p27, p19, p15, and p12, and their size by SDS-PAGE. Monospecificity of each of the antisera was assessed by adsorption with the three respective heterologous gag proteins, followed by immune precipitation of 35S-methionine-labelled proteins of the virion (AMV) lysate. The p15-mediated in vitro cleavage proceeds rapidly and specifically. In the early stages (10 min incubation with p15) the Pr76gag was absent, and an optimum amount of six cleavage intermediates with the following size, composition (shown in parentheses) and orientation (determined form the presence or absence of antigenic determinants of p19 or p15 proteins as N- or C-terminal moieties of the precursor) was found: N-terminal fragments of 66K (p19, p27, p12) and 60K (p19, p27); internal fragments of 37K (p27, p12); C-terminal fragments of 51K (p27, p12, p15), 32K (p12, p15) and 21K (p12, p15). These intermediates were converted into the following four final cleavage products, as the only polypeptides detected after prolonged (5 h) incubations: mature p27 and mature p15 proteins and 38K and 34K polypeptides containing only p19 antigenic determinants. Mature p12 an p19 proteins were not found among cleavage products. Autocatalytic cleavage of Pr76gag was not observed. The results hav allowed us to conclude that the arrangement of the gag proteins in the Pr76gag is N-p19-(p10?)-p27-p12-p15-C and that under in vitro conditions p15 recognizes three correct cleavage sites on native Pr76gag: one located at p12-p15 junction and two on both sides of the p27 moiety, as well as aberrant cleavage sites located inside p12 and possibly also p10 moieties.