SUMMARYControl and bursectomized chicks were i.m. injected with 6 mg of cyclophosphamide (Cy) (0.15 ml in saline) at hatching (Cy1×) or at hatching and the day after hatching (Cy2×). The visual basophilic lymphoid cells of the bursal follicles were replaced by large pale reticular cells in the 2-day-old Cy-treated chicks. No regeneration was observed in the bursas of chicks from the Cy2× group, while basophilic lymphoid cells appeared in the bursa of 2-week-old chicks from the Cy1× group. Germinal centers (GC) in the spleen and cecal tonsil were absent, reduced in number, or numerous in chickens from the Cy2×, Cy1×, and control groups, respectively. The Cy treatment reduced the number of plasmacytic cells. Bursectomy and a single injection of Cy were more effective in depressing the immune response than was either treatment alone. Bursectomy plus Cy1× and Cy2× produced agammaglobulinemia