The influence of different genetic environments on the expression of HLA complex-controlled antigens has been investigated using cell lines with various defects in the synthesis of these molecules and a somatic cell hybrid derived from them. A very sensitive bacterial binding assay allowing simultaneous evaluation of the morphology of a given cell and the quantity of a surface molecule has been developed for these studies. The fetal erythroid cell line K562, the Burkitt's lymphoma-derived cell line DAUDI, and their hybrid DUTKO1 have been employed. K562 and the hybrid, but not DAUDI, expressed HLA-A,B,C heavy chains as detected by the monoclonal antibody W6/32.HL, while two monoclonal antibodies (TU48 and 2BC4) against the supertypic specificities HLA-Bw4 and Bw6 showed no reactivity. The presence of human Ia-like antigens on the cell surfaces was investigated with a panel of eight monoclonal antibodies. K562 cells were completely unreactive, and DAUDI cells gave the expected positive reaction, but about 1% or less of the cells in the DUTKO1 population appeared to express these antigens as well. We discuss possible reasons for the failure to detect HLA-B antigens with monoclonal antibodies and the lack of complete "dominance" of the K562 genome in the hybrid cell line.