An Rc-mutant of Escherichia coli that lacks UDPgalactose 4-epimerase grows normally without galactose but makes lipopolysaccharide lacking most of its carbohydrate. Exogenous galactose overrides the mutation and results in the formation of a complete lipopolysaccharide, thereby producing a smooth phenocopy. The smooth phenocopy was much more resistant to the bactericidal activity of normal human serum than was the rough phenotype. More complement was utilized by the rough mutant in the bactericidal process than by the smooth phenocopy. An antiserum was prepared in rabbits to a specific outer membrane protein in the mutant bacterium, the lambda receptor, whose expression could be suppressed by the addition of 10 mM maltose. The effect of the O-antigen in the lipopolysaccharide produced by the smooth phenocopy on the binding of antibody to the lambda receptor was determined. The smooth phenocopy exhibited significantly less binding of antibody than did the rough phenocopy. In addition, expression of the lambda receptor had little effect on the binding of antibody to the lambda receptor in the smooth phenocopy but caused significantly increased binding in the rough mutant. The results suggest that the increased resistance to the lethal action of normal human serum shown by the smooth phenocopy may be due to the blocking of antibody binding sites by the O-antigen of lipopolysaccharide, thereby preventing activation of the classical pathway of complement.