In chickens from congenic inbred lines CB and CC that differ in the ma
jor histocompatibility complex, we observed significantly different CD
4+:CD8+ ratio in peripheral blood lymphocytes. In CB chickens this rat
io was about 3 and in CC chickens 1.5. They also differ in their respo
nse to Rous sarcoma virus induced tumors. In CB line these tumors are
regressing, in CC line the tumors are progressing.