Chickens were bursectomized in ovo on days 18, 19 or 20 of incubation or within 6 h of hatch and immunized at day 28 after hatch by an intravenous injection of sheep red blood cells (SRBC). The immune response in the blood was measured by haemagglutination tests on the serum and by immunocyto-adherence tests for enumeration of rosette-forming cells with SRBC. Total serum 19S and 7S immunoglobulins were determined by radial immunodiffusion and the number of germinal centres per mm2 of fixed and stained spleen tissue sections was determined. In ovo bursectomy produced undetectably low serum levels of haemagglutinins, a reduction in SRBC rosette-forming cells with normal or increased 19S and reduced 7S immunoglobulins together with a complete disappearance (absence) of germinal centres in the spleen. The role of 19S antibody in the generation of germinal centres is discussed. The finding that birds which are equipped with serum 19S but no 7S Ig lack splenic germinal centres casts doubt on the hypothesis that the switch 19S leads to 7S Ig is necessarily an intrabursal event.