AbstractThe 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 rosetteforming 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.