The influence of antigenic stimulation on cell proliferation in different lymphoid organs of the chicken was investigated, as studied by 3H-thymidine incorporation into DNA. Six-week-old chickens were either immunized with human serum albumin (HSA) or given sterile physiological saline 24 or 72 hours before sacrifice. Thirty minutes before sacrifice, the chickens were intravenously labelled with 3H-thymidine. At 24 hours after the immunization the incorporation of label (cpm/μg DNA) into the bursa of Fabricius was significantly higher in the immunized chickens than in the saline injected controls. At 72 hours after the immunization, the mean incorporation of label into the bursa was of the same magnitude in the immunized and the control chickens. These findings suggest that the bursa of Fabricius, although a central lymphoid organ, may react by an increased cell proliferation early after antigenic stimulation. The incorporation of label into the thymus was not at any time influenced by the immunization.