In this study, changes in some non-specific immune defense mechanisms of rainbow trout (On-corhynchus mykiss) following exposure to aqueous extracts of Nigella sativa seeds were inves-tigated. A total of 900 fish (mean weight 102.3g) were used in the study. Fish were treated with black seed oil using one of three different application techniques. The first method involved feeding the fish with 0.1%, 1.0% and 10% dilution of black seed oil over a 21d period. In the second method, 0.1%, 1.0% and 10% dilution of black seed oil was injected to the fish. In the third method, 0.1%, 1.0% and 10% dilution of black seed oil were applied directly onto the lat-eral line of fish (‘smearing’) over a 3d period. Blood samples of fish were taken at 3, 7, 14 and 21d post-exposure and hematocrit, leucocrit, erythrocyte, leucocyte, nitroblue tetrazolium ac-tivity, protein level and total immunoglobulin levels were determined. The highest levels of hematocrit, erythrocyte, leucocyte and nitroblue tetrazolium activity occurred in fish exposed using the smearing method. Differences in leucocrit levels of groups exposed to the three methods were not significant. The highest levels of total protein occurred in fish exposed via feeding. The highest levels of total immunoglobulin occurred in fish exposed via injection and decreased suddenly. Nonetheless, the smearing method caused regular increase and decrease. It was observed that black seed oil caused increase of fish protein content, even at the lowest dose in fish exposed via feeding. In this study, the effects of smearing method were investi-gated on immune system of rainbow trout, as well as, determined differences of feeding and in-jection methods on immune response than smearing method.