AbstractChickens injected variously withMycoplasma gallisepticumorganisms or sheep erythrocytes and assayed for specific serum agglutinins were individually evaluated with respect to lymphoid development in spleen, thymus, cecal tonsils, cloaca, and bursa of Fabricius. The examinations were carried out during the period of normal maximal bursa size on chickens which had been chemically bursectomized (CBx) at the 7‐day‐embryo stage and on intact controls. The embryonic genesis of bursal epithelium was completely prevented in 41 of 43 CBx birds. These bursaless birds nonetheless collectively formed agglutinins in 1 of 30 cases at 10 – 12 days after primary stimulation, in 6 of 26 cases at 22 – 23 days after primary stimulation, and in 15 of 32 cases after secondary stimulation. The titers compared favorably with controls. Agglutinin responsive bursaless birds had secondary follicles in spleen and cecal tonsils and an impressive, if variably subnormal, development of lymphoepithelial tissue. By contrast, agglutinin negative bursaless birds lacked germinal centers and had only moderately to sparsely populated lymphoepithelia. This lymphocyte deficit was especially marked in the proctodeal and urodeal roofs and cecal tonsils. Plasmacytes in bursaless birds were rarely found in spleen but were always found in gut lymphoepithelia in numbers correlating with regional lymphoid cell