Monoclonal monomorphic as well as polyclonal antibodies against H-B class I (B-F) antigen were used to evaluate the expression of these molecules on normal and RSV-transformed chick embryo fibroblasts. The results indicate that chicken fibroblasts transformed in vitro by the Schmidt-Ruppin strain Rous sarcoma virus of subgroup B (SR-RSV-B) present reduced H-B class I antigen expression as compared to uninfected cells of the same inbred strain. This quantitative reduction was observed at the cell membrane and in whole cell lysates. Chick embryo fibroblasts infected with avian leukosis virus RAV-1 show that the levels of H-B class I antigens were indistinguishable from the levels measured on the control cells. Normal embryo fibroblasts derived from the inbred lines B4/B4 express higher levels of H-B antigens than normal cells carrying the B12/B12 genotype.