A new antigenic system in the rat homologous to the Qa/TL antigen system in the mouse has been characterized. It was detected by antibodies raised in donor-recipient combinations that were matched for the RT1.A, B, D, E loci in the major histocompatibility complex (MHC): (R11 X BN)F1 anti-BN.1L(LEW), (R18 X BN)F1 anti-BN.1L, and BN.1LV1(F344) anti-BN.1L. Absorption analyses using these antisera and a variety of inbred, congenic, and recombinant strains identified three alleles, RT1.Ga, Gb, Gc, of which Gc is a null allele. The strain distribution of these alleles was determined using 37 strains of rats representative of all of the prototypic haplotypes and a number of congenic and recombinant strains. The use of the congenic and recombinant strains showed that the RT1.G locus was linked to the MHC and that the most probable gene order was A-E-G. Testcross analysis showed that the map distance between A and G was 1.4 cM (4/285 recombinants). The RT1.G antigen has a heavy chain of Mr 46,000 and is present on both T and B cells.