One-day-old chickens treated via drinking water with kitasamycin (0.7 g/l for 6 days then 0.35 g/l for 15 days) or with chloramphenicol (1 g/l for 6 days the 0.5 g/l for 15 days), were immunized at the 24th day with sheep red blood cells. Body and spleen growths were recorded and compared to control every week for 5 weeks after immunization. Humoral and cell-mediated immune responses were measured by hemagglutinating antibodies titration, direct and indirect plaque forming cells (PFC) numeration, and graft versus host reaction (GVHR). Both antibiotics reduce antibody response and number of PFC, chloramphenicol being significantly more suppressive than kitasamycin. A short stimulation of cellular response is shown with kitasamycin, which stimulates GVHR, whereas chloramphenicol has a negative effect.