We have cloned and sequenced the d15 gene from two strains of Haemophilus influenzae type b (Hib) and two strains of nontypeable H. influenzae (NTHI). The nucleotide and deduced protein sequences of d15 are highly conserved, with only a small variable region identified near the carboxyl terminus of the protein. Analysis of upstream sequences revealed that the H. influenzae d15 gene may be part of a large potential operon of closely spaced open reading frames, including one with significant homology to the Escherichia coli cds gene encoding CDP-diglyceride synthetase. Southern blot analysis demonstrated that the d15 gene is also present in H. influenzae types a, c, d, e, and f and in Haemophilus parainfluenzae. A recombinant D15 (rD15) protein was expressed in good quantity in E. coli from the inducible T7 promoter, and monospecific anti-rD15 antibodies were raised. Immunoblot analysis of H. influenzae serotypes a, b, c, d, e, and f, NTHI, and H. parainfluenzae lysates revealed that they all expressed a cross-reactive D15-like protein. Purified rD15 was found to be highly immunogenic in mice, guinea pigs, and rabbits, and passive transfer of anti-rD15 antibodies protected infant rats from challenge with H. influenzae type b or type a in infant rat models of bacteremia. Thus, D15 is a highly conserved antigen that is protective in animal models and it may be a useful component of a universal subunit vaccine against Haemophilus infection and disease.