RTF1 is the the major histocompatibility complex (MHC) in the rat which contains genes that code for two class II histocompatibility antigens. RT1-B is an antigen of the RT1 complex. It is a protein dimer consisting of an alpha and beta glycoprotein chain and is homologous to I-A and I-E genes, respectively, in the H-2 complex of the mouse. MHC Class II antigens are useful in studying T helper cell interaction with class II positive antigen presenting cells (dendritic cells, B cells, macrophages) and offer new possibilities for studying the development of T helper cells since these antibodies also stain stromal cells in the thymus.