The processing of gag and gag-pol polyproteins by human immunodeficiency virus type 1 (HIV-1) protease is a crucial step in the formation of infectious HIV-1 virions. In this study, we examine whether particles produced in the presence of inhibitors of HIV-1 protease can subsequently undergo gag polyprotein cleavage with restoration of infectivity following removal of the inhibitors. Viral particles produced during 7 days of culture in the presence of the protease inhibitors KNI-272 (10 microM) and saquinavir (5 microM) contained predominantly p55gag polyprotein but little or no p24gag cleavage product. Following resuspension of the particles in medium free of the inhibitor, some gag polyprotein processing was detected in particles produced from the KNI-272-treated cells, but not from the saquinavir-treated cells within the first 3 h. However, the majority of the protein remained as p55gag throughout a 48-h experimental period. The infectivity (50% tissue culture infective dose per milliliter) of the viral particles from KNI-272-treated cells was 10(6)-fold lower than that of control particles and did not significantly increase over the 48 h after the inhibitor was removed, despite the apparent return of protease function in a subset of these virions. This failure to restore infectivity was due neither to a reduction in the number of particles produced by protease inhibitor-treated cells nor to a failure of HIV RNA to be packaged in the virions. These particles also failed to express the mature phenotype by electron microscopy. Thus, while some processing of the gag polyprotein can occur in isolated HIV virions, this does not appear to be sufficient to restore infectivity in the majority of particles. This finding suggests that there may be constraints on postbudding polyprotein processing in the production of viable particles. These results should have positive implications regarding the use of protease inhibitors as anti-HIV drugs.