The chicken MHC-B locus affects the response to several strains of Rous sarcoma virus (RSV). We evaluated the association between haplotypes of the MHC-B locus and responses to the J strain of RSV by using an F(2) experimental resource family constructed with tumor-regressive (White Leghorn) and tumor-progressive (Rhode Island Red) chickens. The MHC-B haplotypes were determined by genotyping of the microsatellite marker LEI0258 and MHC-B locus class I alpha chain 2 (BF2). Two haplotypes in the resource family, one associated with tumor regression and one with progression, were defined by these 2 markers. To discriminate more precisely the regressive haplotype in this family, we further developed 35 SNP markers at the MHC-B locus. Information on the haplotypes revealed here should be useful for identifying chickens with regression and progression phenotypes of J-strain RSV-induced tumors.