Nontypable Haemophilus influenzae are Gram-negative bacilli that represent a common cause of human disease. These organisms initiate infection by colonizing the upper respiratory tract. Despite the essential role of colonization, the bacterial determinants of this process remain poorly defined. We recently identified a family of surface-exposed high-molecular-weight proteins of nontypable H. influenzae that are related to filamentous hemagglutinin, a critical adherence factor of Bordetella pertussis. The genes encoding the two such high-molecular-weight proteins (HMW-1 and HMW-2) expressed by a prototypic nontypable H. influenzae strain have now been cloned and sequenced. In this study we examined the role of the HMWs in adherence to human epithelial cells. We found that loss of expression of HMW-1 by the prototypic strain and a HMW-1-like protein in a heterologous nontypable H. influenzae strain markedly decreased the capacity to adhere. The absence of expression of both HMW-1 and HMW-2 in the prototypic strain or their homologs in the second strain was associated with a further decrease in adherence. Expression of either HMW-1 or HMW-2 in nonadherent laboratory strains of Escherichia coli resulted in acquisition of the adherence phenotype. These results indicate that both HMW-1 and HMW-2 and the homologous proteins from a second strain can mediate attachment. We speculate that these proteins and the related proteins in other nontypable H. influenzae isolates are important colonization factors.