Twelve congenic lines of White Leghorn chickens carrying distinct haplotypes of the MHC (B blood group) but sharing a common genetic background from a highly inbred line (UCD 003) were compared. Each of the lines had been bred back to Line UCD 003 for five generations before intercrossing its members to establish its distinct genetically homozygous MHC type. Seven generations of these congenic lines were compared between 1981 and 1987 for fertility, hatchability of fertile eggs, egg production, mortality to 40 wk of age, 40-wk egg weight, and 40-wk body weight. Statistically significant differences between MHC haplotypes were found for all traits except fertility.