Minimum inhibitory concentrations of 13 beta-lactam antibiotics (ampicillin, amoxycillin, amoxycillin/clavulanate, imipenem, cefazolin, cefadroxil, cefaclor, cefuroxime, cefotaxime, cefepime, cefpirome, cefpodoxime and cefixime), were determined for 76 strains of beta-lactamase negative Haemophilus influenzae, isolated over a five year period (1985-1990) that gave reduced zones to cefuroxime on disc testing when compared to the control strain H. influenzae NCTC 11931. MIC90 values for all antibiotics (except imipenem) were approximately ten times higher than the MIC90 values for a susceptible control group. Increased resistance was not associated with any particular biotype, although three biotype III strains were highly resistant to imipenem. More than 50% of strains with reduced susceptibility to beta-lactam antibiotics were isolated from patients with chronic respiratory disease. Published data on the sputum concentration of each antibiotic were compared to the MIC90 values obtained for the susceptible and resistant strains.