In the course of a multicentric surveillance study, nine laboratories sent 375 isolates of Haemophilus influenzae to the Sector de Resistência aos Antibióticos (SRA) from the National Institute of Health in Lisbon, between 1 January and 31 December 1992. The majority of the H. influenzae isolates were from the respiratory tract (84.8%); only 5.1% were of invasive origin. Overall resistance for ampicillin was 11.7%, tetracycline 3.7%, and chloramphenicol 2.4%. All isolates tested were fully susceptible to cefotaxime, ceftriaxone, ciprofloxacin and rifampicin. Multiresistance was rare, occurring only in 2.4% of the isolates, although 50% of the ampicillin resistant strains had at least one additional resistance marker. Forty two isolates (11.2%) produced a TEM-1 type beta-lactamase, as shown by isoelectric focusing. beta-lactamase production was not detected in two of the ampicillin resistant strains. Fifteen of the 42 beta-lactamase producing strains (35.7%) contained detectable DNA plasmid: nine harboured large plasmids with an apparent molecular mass of 45 or 54 kb depending on their resistance phenotype and six harboured a small plasmid of 5 kb. In order to study transfer of resistance in both ampicillin and multiresistant strains conjugation experiments were performed for 14 isolates, seven of which harboured a large plasmid and seven had no detectable plasmid DNA. All 14 transferred their resistance phenotype but only a single large plasmid could be demonstrated in ten transconjugants. Restriction endonuclease analysis of plasmids from six representative transconjugants, isolated in different hospitals, revealed that there was no dissemination of a single R plasmid, which suggests an independent process of acquisition of resistance genes.