Two distinct beta-lactamases have been isolated from Moraxella catarrhalis: the stronger acting BRO-1 enzyme and the weaker acting BRO-2. Several reports have noted an effect of penicillin and ampicillin on infections caused by M. catarrhalis in spite of the presence of beta-lactamase production. The purpose of this work was to charaterize the beta-lactamases of M. catarrhalis isolated from Danish children regarding type and susceptibility, and to relate these findings to the eradication of beta-lactamases-producing strains by use of antibiotic treatment with penicillin or ampicillin. MICs for penicillin V, ampicillin, cefuroxime and amoxicillin/clavulanic acid (2:1) were determined in 70 strains of M. catarrhalis: 46 strains from children with lower respiratory tract infection (LRTI) and 24 strains from respiratory healthy children, beta-lactamase production was found in 59 strains. The BRO-1 enzyme was identified by isoelectric focusing in 55 strains (93.2%) and BRO-2 in 3 strains (5.1%); in 1 strain no isoelectric bands were produced. All strains were susceptible to cefuroxime and amoxicillin/clavulanic acid, and non-beta-lactamase-producing strains were susceptible to penicillin and ampicillin. For the beta-lactamase-producing strains, MIC50 of penicillin was 8.0 micrograms/ml, while MIC50 of ampicillin was 1.0 microgram/ml and MIC90 of ampicillin was 2.0 micrograms/ml. M. catarrhalis was more often eradicated from the children who received antibiotic treatment with penicillin or ampicillin than from those who did not receive any treatment, indicating an in vivo effect of penicillin and ampicillin in spite of the beta-lactamase production.