A protein from H. influenzae designated NucA is isolated and purified. The NucA protein has the amino acid sequence of amino acids 26-603 of SEQ ID NO:2 or a biologically equivalent amino acid sequence thereof. Amino acids 1-25 of SEQ ID NO:2 are the signal peptide, which is cleaved during processing of the mature protein. The NucA protein has a molecular weight of approximately 63,000 Daltons as measured on a 12% SDS-PAGE gel and possesses 5'-nucleotidase activity. The NucA protein is obtained by isolation and purification from the H. influenzae organism, by chemical synthesis or by recombinant expression by an isolated and purified nucA DNA sequence which encodes the NucA protein. Such a DNA sequence hybridizes under standard high stringency Southern hybridization conditions with a DNA sequence encoding the NucA protein of H. influenzae having the amino acid sequence of amino acids 26-603 of SEQ ID NO:2 or a biologically equivalent amino acid sequence thereof. The NucA protein is used to prepare a vaccine composition which elicits a protective immune response in a mammalian host to protect the host against disease caused by H. influenzae.