Haemophilus influenzae organisms, both type b and non-typable strains, were tested for their adherence to human buccal epithelial cells. Adherence was determined by visual examination of gram-stained specimens of epithelial cells following incubation with H. influenzae. Among 36 isolates tested, 2 organisms adhered well to buccal cells; a type b nasopharyngeal isolate showed a mean of 19.94 bacteria per cell and a non-typable throat isolate showed a mean of 37.72 bacteria per cell. These isolates readily hemagglutinated human red blood cells. None of the other 12 type b blood or spinal fluid isolates tested adhered well (range 0.04-1.34 bacteria per cell) nor hemagglutinated red cells. There was no difference in the adherence of H. influenzae to buccal mucosal cells from 6 donors, and no difference between epithelial cells obtained from the buccal or nasal mucosa. Although both type b and non-typable H. influenzae strains appear capable of adhering to human epithelial cells, many strains do not. The role of adherence factors in the maintenance of mucosal colonization with these poorly-adherent strains remains unknown.