The effects of aflatoxin B₁ (AFB₁), aflatoxin M₁ (AFM₁), deoxynivalenol (DON) and zearalenone (ZEA) on the viability, chemiluminescent (CL) response and expression of cytokine mRNA of bovine neutrophils (PMNs) were evaluated. The opsonized zymosan (OPZ)-stimulated CL response of PMNs was significantly (P<0.05) decreased by AFB₁ (>50 pg/ml), AFM₁ (>50 pg/ml) and ZEA (>50 pg/ml). The phorbol myristate acetate (PMA)-stimulated CL response PMNs was significantly (P<0.05) decreased by AFB₁ (>0.5 pg/ml), AFM₁ (>50 pg/ml), ZEA (>500 pg/ml) and DON (>5 pg/ml). Treatment with AFB₁ resulted in reduction in the mRNA expression of interleukin-1β and tumor necrosis factor-α of PMNs stimulated with OPZ and PMA. These results suggest that these four mycotoxins have inhibitory effects on the function of bovine PMNs.