Nigella saliva (black seed) is a plant used on large scale for many purposes. The prophylactic and treatment effects of the oil extracted from this plant were studied on Pneumocystis carinii. For this purpose, 48 male albino rats weighing each 100 - 120 g were used. The induced experimental infection with Pneumouystis carinii was carried out by im­munosuppression of rats with 2 mg/I dexamethasone sodium phosphate together with 500 mg/I tetracycline hydrochloride in drinking water. Rats acquired the infection with P. carinii 5 - 6 weeks post­immunosuppression. Macroscopical examination of lungs was carried out, and stained smears with Giemsa stain were also examined. The results were analysed statistically. The immune enhancing effect was clear in both prophylactic and treated rats. In case of prophylactic and treat­ment studies, the percentages of both survival prophylactic-infected rats (40%) and survival infected-treated ones (73.3%) were compared to the survival infected-control rats (0%) and survival control non-infected non-treated rats (100%). The percentage of the survived rats in both prophylactic and treated groups that showed no infection with P. carinii developmental stages in Giemsa stained lung smears, were 66.7% and 27.3% respectively, although, the percentage of other survived rats in the same 2 groups that showed moderate infection were 33.3% and 72.7% respectively.