Escherichia coli K100 is known to induce antibodies cross-reactive with the capsular polysaccharide (CP) of Haemophilus influenzae b (Hib); the cross-immunogenicity is found consistently in a number of mammalian species including man. We have studied the reciprocal cross-immunogenicity of Hib or its purified CP in man. Of 25 adults and 13 children responding to systemic immunization with Hib CP, only four and four, respectively, made K100 CP cross-reactive antibody, determined by radioantigen-binding inhibition. The detection of preimmunization cross-reactive antibody was not statistically predictive of induction of postimmunization cross-reactive antibody. Cross-reactive antibody was detected in only one of 11 children making an anticapsular response to Hib systemic infection and two of four children responding to asymptomatic nasopharyngeal colonization. In contrast, cross-reactivity was detected in the naturally occurring anti-Hib-CP antibody of 27 of 42 children. Thus, humans responding to Hib or its CP often do not make antibody to the K100-cross-reactive determinant(s), implying that the cross-reactive natural antibodies frequently seen in children must have a stimulus other than Hib.