The present invention is a mutant retroviral protease which confers an increase in retroviral stability. Retroviruses expressing the instant mutant retroviral protease exhibit at least a 2-fold increase in infectivity half-life as compared to wild-type retrovirus. Unexpectedly, a Gly119Glu mutation in the protease enhances retroviral stability in the presence of various wild-type envelope proteins including wild-type amphotropic, ecotropic and 10A1 murine leukemia viruses. The improved stability of the mutant retrovirus leads to more facile virus production and enhanced infection efficiency.