The spectrum of reactivity with various types of cells of a monoclonal antibody (CH-4) which detects a private MHC antigen of chickens was analysed. CH-4 agglutinates only RBCs that possess the B2 (MHC) haplotype. A new rosette-forming cell (RFC) assay was devised to detect individual cells (excluding RBCs) that possess the CH-4 specificity on their cell surfaces. RBCs that have CH-4 chemically coupled to their surfaces attach to, and form rosettes with, B2 antigen-bearing cells. Most non-RBC RFC were detected in active erythropoietic organs (adult bone marrow and embryonic spleen), and none were found in organs where erythropoiesis does not occur: adult thymus and bursa. Preincubation of bone marrow cells with CH-4 plus complement almost completely inhibits their capacity to form CFU-E without affecting their ability to form GM-CFU. In addition, CH-4 plus complement does not inhibit the capacity of B2/B2 lymphocytes to induce a graft-versus-host reaction under conditions where anti-B2 lymphocyte alloantisera are completely inhibitory. Our results strongly suggest that CH-4 monoclonal antibodies detect a private specificity on a gene product of the B-G locus whose expression is restricted to erythroid stem cells and erythrocytes.