Technical Abstract: Poultry diets often are supplemented with various antibiotics and anticoccidial drugs as prophylactic measures against infection and to improve growth. To understand whether continuous exposure of birds to antibiotics, anti-coccidial drugs, and growth promoters would affect innate immunity, the in vitro effects of some commonly used supplements on heterophil functions was examined. The supplements tested were bacitracin, chlortetracycline, roxarsone, monensin, salinomycin, and tylosin. The effects of these supplements on viability, respiratory oxidative burst, interleukin-6 (IL-6), and gelatinase production was examined using heterophils that were activated with bacterial lipopolysaccharide (LPS) and killed Staphylococcus aureus (SA) cells. Both monensin and salinomycin reduced heterophil viability. Except roxarsone all other drugs partially inhibited respiratory oxidative burst activities induced by LPS/SA. The LPS/SA-stimulated IL-6 production was similarly affected. The gelatinase production was constitutive and was only marginally increased due to LPS/SA which was minimally affected by different drugs. In conclusion, it appears that heterophil function may be affected by antibiotics and anticoccidial drugs.