In this study, effects ot chloramphenicol on phagocytosis and intracellular killing acllvity of polymorphonuclear leucocytes (PMNLs) were investigated. This study was performed in vitro and in vivo conditions. Nine Holstein-Fresians cows (8-13 months' ages) were used. Chloramphenicol were tested at 4000, 1000 and 10 ^ig/ml ol incubation mixtures. Chloramphenicol had no effect on PMNL functions at 1000 and 10 pg/ml concentrations. However, chloramphenicol caused significant increase in intracellular number of live C. albicans on per PMNL at the intermediate dose The activities of PMNLs were also depressed by chloramphenicol at the highest dose. In vivo studies, at 12 hours intervals, each cow were treated by intramuscularly administration with chloramphenicol at recommended dose (10mg/kg bw) lor 5 days. Blood samples were collected at 2.5hr after first injection. A significant reduction in the percentage of phagocytosis and intracellular killing activity was observed beginning of the first day.