The susceptibility to antibiotics of three respiratory pathogens, Haemophilus influenzae, Streptococcus pneumoniae and Moraxella catarrhalis, was determined. The isolates were obtainied in three regional laboratories in the Netherlands and tested using the microdilution method. After incubation the turbidity was measured in a fotometer and the results were analysed at the RIVM by a computer program. Not all isolates grew well in the broth medium chosen and the resistance of a small number of strains retested on agar, was not confirmed. So the broth medium still needs improvement and the method must be compared with the established agar-dilution technique. Nevertheless the results of the investigation confirmed those of earlier research. Most isolates were still sensitive to commonly used antibiotics. The known insensitivity of Haemophilus influenzae to erythromycin and the marginal susceptibility of Streptococcus pneumoniae to quinolones was confirmed. Resistance of Haemophilus influenzae to amoxycillin was found in 10% of the isolates, mostly due to beta-lactamase production. Reduced susceptibility of Streptococcus pneumoniae to penicillin was found in approximately 2% of the isolates.