Cell suspensions from the spleen of normal and hormonally-bursectomized chickens were reacted with 125I-labelled anti-immunoglobulin proteins of different class specificity. Following autoradiography, it was demonstrated that 22.5%, 24.3% and 11.8% of normal spleen cells labelled with radioiodinated anti-light chain, anti-μ chain, and anti-γ chain, respectively. The efficacy of bursectomy was confirmed by the absence of in vitro immunoglobulin synthesis by spleen fragments. No labelling was observed with any anti-immunoglobulin protein on spleen cells from bursectomized birds. Since maternal serum immunoglobulins were still present in the bursectomized birds, the possibility of cytophilic antibody being a cause of normal cell labelling was remote. The results support the hypothesis that the bursa-derived (B) cell is the equivalent of the mammalian nonthymus-derived (B) lymphocyte since both have a high surface density of immunoglobulin.