- The effect of paromomycin sulfate (PR) on the respiratory burst response (RB) of C3H/HeJ mouse peritoneal macrophages infected with Leishmania major , LRC-L137, was studied in vitro . Giemsa staining and acridine orange staining combined with nitro blue tetrazolium (NBT) reduction reaction were used to determine parasite survival and RB response at the level of individual cells. RB response was detected in only 18% of the infected macrophages compared with 60% of the control, uninfected cells. Only 7% of the total RB-responder (NBT-positive) cells showed the presence of intracellular amastigotes. PR at 100 μg/ml reduced the number of the infected cells by 85% on the 4th day of cultivation. PR at the same concentration restored 50% of the RB activity of these cells. PR given alone had no effect on the RB of the uninfected macrophages. A similar effect of PR on RB activity was observed with PMA-stimulated macrophages. Both Leishmania -infected and uninfected macrophages treated with PR (100 (μg/ml) for 4 days retained 50% of their leishmanicidal activity upon reinfection due to drug accumulation within the cells.