The current prevalence of ampicillin-resistant Haemophilus influenzae b meningitis requires accurate knowledge of susceptibility to alternative antibiotics. One variable affecting susceptibility is inoculum size. We studied the susceptibility of 200 clinical isolates of H. influenzae b to ampicillin, carbenicillin, and cefamandole at inocula of 10(5) and 10(7) CFU by two techniques. Fifty ampicillin-susceptible and fifty ampicillin-resistant strains were tested for susceptibility to ampicillin by broth dilution while 100 of each were tested by agar dilution. An inoculum effect was found, being greatest with the ampicillin-resistant strains. The range of minimal inhibitory concentrations for the resistant strains was 25 to 800 microgram of ampicillin per ml at an inoculum of 10(5) and 2,000 to less than 6,000 microgram of ampicillin at 10(7); 1.0 to 150 microgram of carbenicillin per ml at 10(5) and 6.2 to 2,000 microgram of carbenicillin per ml at 10(7); 0.4 to 2.0 microgram of cefamandole at 10(5) and 1.0 to 125 microgram/ml at 10(7). Because of this inoculum effect, we would not recommend the use of carbenicillin or cefamandole for therapy of ampicillin-resistant H. influenzae meningitis.