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IT has long been known that many strains of gram-negative bacteria are insensitive to the bactericidal action of normal human serum despite the presence in the serum of bactericidal antibody and complement (Georgevitch, 1926; Mackie and Finkelstein, 1931); the nature of this resistance remains obscure. Serum resistance in Escherichia coZi has been variously ascribed to the amount of cell-wall lipopolysaccharide (LPS), the length and number of LPS 0-side chains and to both the amount and haemagglutination-inhibiting activity of acidic polysaccharide K antigen (see review by Inoue, 1972). In a recent study (Taylor, 1975), it was found that none of these factors was able to explain the behaviour of serum-resistant mutants derived from smooth, E. coli strains isolated from the urinary tract, and it was suggested that these mutants synthesised an additional surface component, possibly protein in nature, which was responsible for serum resistance. The present study concerns the analysis of one of the serum-resistant mutants and the corresponding parent strain by gel-diffusion techniques an