New arrangements of chicken major histocompatibility complex (MHC) class I BF and class IV BG genes are created through recombination. Characterizing the immune responses of such recombinants reveals genes or gene regions that contribute to immunity. Inbred Line UCD 003 (B17B17) served as the genetic background for congenic lines, each containing a unique MHC recombinant. After an initial cross to introduce a specific recombinant, 10 backcrosses to the inbred line produced lines with 99.9% genetic uniformity. The current study compared Rous sarcoma virus (RSV) tumor growth in 5 congenic lines homozygous for MHC recombinants (003.R1 = BF24-BG23, 003.R2 = BF2-BG23, 003.R4 = BF2-BG23, 003.R5 = BF21-BG19, and 003.R13 = BF17-BG23). Two experiments used a total of 70 birds from the 5 congenic lines inoculated with 20 pock forming units of RSV subgroup C at 6 wk of age. Tumor size was scored 6 times over 10 wk postinoculation followed by assignment of a tumor profile index (TPI) based on the tumor size scores. Tumor growth over time and rank transformed TPI values were analyzed by least squares ANOVA. Tumor size increased over the experimental period in all genotypes through 4 wk postinoculation. After this time, tumor size increased in Lines 003.R1, plateaued in Lines 003.R2, 003.R4, and 003.R13, and declined in 003.R5. Tumor growth over time was significantly lower in Line 003.R5 compared with all other genotypes. In addition, Line 003.R5 chickens had significantly lower TPI values compared with Lines 003.R2, 003.R4, and 003.R13. The TPI of Line 003.R1 did not differ significantly from any of the other genotypes. The BF21 in Line 003.R5 produced a greater response against subgroup C RSV tumors than did BF24, found in 003.R1; BF2 found in 003.R2 and R4 as well as BF17 found in 003.R13.