Recombinant human (rh)-prorenin expressed using the baculovirus expression vector system (BEVS) is processed in situ to produce active rh-renin. However, rh-renin is significantly degraded during very late stage of the infection. This study aims at preventing excess degradation of recombinant proteins in the BEVS. Culture media of baculovirus-infected Sf9 insect cells are supplemented with either protease inhibitors or bovine serum albumin (BSA) at various time points postinfection. Although the degradation of active rh-renin is suppressed by cysteine protease inhibitors, proteolytic activation of rh-prorenin into active rh-renin is also inhibited concurrently, and the yield of active rh-renin is decreased. On the contrary, BSA supplementation during late stages of the infection is useful for preventing the degradation of active rh-renin without affecting the proteolytic activation. This indicates that addition of proteins to culture media at specific time points postinfection is a simple and effective method for suppressing the degradation of recombinant proteins expressed by the BEVS.