Comparisons of O- and K-antigenic types and serum sensitivity were carried out with 149 strains of Escherichia coli isolated from adults with bacteremia and 46 strains from neonates with bacteremia. O-antigenic types O6, O4, O2, O16, O18, and O7 were observed most frequently, but their relative prevalence did not differ materially between adult and neonatal bacteremias. A greater proportion of strains from neonatal bacteremia contained K1 antigen and were autoagglutinable compared with strains from adult bacteremia, although K1 was the most frequent K antigen found in strains from adults. K-antigen-containing strains did not appear to be associated with enhanced severity of bacteremia, but nontypable strains, auto-agglutinable strains, and strains of O-antigenic types O4, O6, and O8 were associated with a greater frequency of shock and fatal outcome in adults. No differences could be detected between the serum sensitivities of E. coli isolated from adult bacteremia and those from neonatal bacteremia. K antigen did not affect serum sensitivity, but E. coli strains of O types O18, O2, O4, and O7 were more serum-resistant than other E. coli. Bacteremia caused by serum-sensitive E. coli was less often associated with shock and death than bacteremia caused by serum-resistant E. coli.