White leghorn chickens, thymectomized and sublethally X-irradiated at hatching, showed increased titers of serum hemagglutinins to sheep erythrocytes (SRBC). Increased numbers of direct spleen plaque forming cells (PFC) were demonstrated with the Jerne technique using this antigen. Deliberate immunization i.v. with SRBC did not significantly increase the hemagglutinin titers or the numbers of direct or indirect PFCs per spleen. Spleen cells from thymectomized-irradiated chickens (TX) in tissue culture displayed impaired proliferative responses, measured as 3H-thymidine uptake, to phytohemagglutinin (PHA) and allogeneic spleen cells treated with mitomycin C. Also the proliferative response of spleen cells from immunized TX chickens to SRBC appeared deficient.