The number of the nontypeable Haemophilus influenzae (HiN) organisms that adhered to the primary mouse fetal lung cells was significantly more than type b Haemophilus influenzae (Hib) organisms. The average number of HiN organisms adherent to host cells was 2,291/100 host cells (range, 1,654-3,182), but that of Hib was markedly reduced to 147/100 host cells (range, 102-238). In this case, P value was< 0.05 by using a paired Student t-test. The sonicated extract from HiN TMS11 organisms inhibited adherence of H. influenzae TMS11 organisms to monolayer at 76.3% and it inhibited adherence of Hib TMS24 organisms at 92.3%. This result indicates that a mediator existing on the surface of HiN organisms may be the same as that on type b organisms. The number of detected organisms in broncho and lung tissues 3 days after intranasal infection with HiN strains was significantly greater than that in infection with Hib strains. Therefore, in vitro adhesive capacity of H. influenzae organisms was correlative to infectivity by intranasal injection.