We have studied the susceptibility to ampicillin and the characteristics of the beta-lactamase activity of the 169 Haemophilus spp. strains (128 H. influenzae, 40 H. parainfluenzae and one H. paraphrophilus) isolated during 12 months, years 1988-1989, in the Hospital del Mar clinical microbiology laboratory. Our objective was to know in the strains of our center the frequency of those resistant or slightly susceptible to ampicillin, the role of beta-lactamases in the loss of susceptibility and the type of enzymes involved. Susceptibility was studied by diffusion for all the antibiotics tested and also confirmed by dilution for ampicillin and other beta-lactam antibiotics. Beta-lactamase production was identified by nitrocefin hydrolysis. The isoelectric point of the beta-lactamase and the identification of their types were determined by analytic isoelectric focusing. The presence of the codifying gene of the TEM-1 enzyme was studied by hybridization with a TEM-1 probe. Of the H. influenzae strains 35 were ampicillin resistant, one moderately susceptible and of the H. parainfluenzae strains six were resistant and two moderately susceptible; all were susceptible to the combination of amoxicillin/clavulanic acid. The ampicillin-resistant strains were beta-lactamase producers. In 40 strains, by isoelectric focusing, the type TEM-1 was identified and the hybridization was positive; in one H. parainfluenzae strain a beta-lactamase of pl 5.8 was observed and the hybridization with TEM-1 probe was negative. The three strains moderately susceptible to ampicillin did not produce beta-lactamase.