Opacity-associated protein A (OapA), which is responsible for the transparent-colony phenotype of Haemophilus influenzae, has been implicated in the colonization of the nasopharynx in an infant rat model of carriage. In this report, we show that OapA mediates attachment to Chang epithelial cells examined by using genetically defined type b and nontypeable H. influenzae strains with or without OapA. We also showed that OapA was conserved among H. influenzae strains by comparing deduced amino acid sequences. Both recombinant OapA and polyclonal anti-OapA antiserum blocked the binding of H. influenzae to Chang epithelial cells, suggesting that the interaction of H. influenzae is specific to OapA. Moreover, the binding of recombinant OapA to epithelial cells further provided evidence that OapA can promote attachment of H. influenzae. Expression of oapA gene in a nonadherent Escherichia coli strain significantly increased the binding to Chang epithelial cells, and disruption of the oapA gene with kanamycin resistance cassette insertion resulted in a significant loss of binding. These findings demonstrate that OapA plays a role in H. influenzae binding to human conjunctival epithelial cells.