HLA antigen‐loss mutants and class I gene transferents were used to analyze the influence of class I expression on natural killer (NK) cell‐mediated lysis. Only HLA antigen‐loss mutants that had lost expression of either HLA‐A and HLA‐B antigens (mutant .184) or of HLA‐A, B and C antigens (mutant .221) were distinctly susceptible to NK‐mediated lysis. Mutants with reduced expression of class II antigens but unaltered expression of class I antigens remained resistant to NK lysis. A direct demonstration of the effect of class I antigen expression on human cells was made by analyzing a variety of gene transferents of the HLA‐A, B, C null mutant .221 expressing only one transferred HLA‐A, B or C gene. These results specifically show that expression of class I antigens, with a possible preferential effect of HLA‐B expression, reduces the susceptibility of mutant .221 to NK‐mediated lysis.