Effect of surgical bursectomy on antibody response was studied in chicks injected intrabursally with sheep red blood cells (SRBC) andBrucella abortuson day 2 of life and challenged intravenously with the same antigens on day 9. Bursectomy performed 2 days after the priming resulted in a significantly decreased anti‐SRBC plaque‐forming cell (PFC) response, whereas bursectomy performed 4 or 5 days after the priming clearly enhanced the PFC response. The enhancing effect of bursectomy on the PFC response was also observed if the priming was made intravenously. In antibody titers against SRBC the enhancing effect was observed only if a secondary and tertiary challenge was performed at the age of 28 and 36 days, respectively. Titers ofBrucellaantibodies were either reduced by the bursectomy (performed 2 or 4 days after the intrabursal priming) or left unaffected (bursectomy performed 5 days after the priming). These findings are discussed in relation to bursa‐derived suppressor