The evolution of antibiotic resistance was studied among common respiratory tract pathogens in five countries of the European Union and in the USA during 1992-1993. The data obtained from a collaborative surveillance study were submitted to population analysis, to detect possible shifts in antibiotic susceptibility and, therefore, associated mechanisms of resistance. Among the emerging haemophilus influenzae phenotypes were isolates that did not correspond to the beta-lactamase negative, amino-penicillin resistant (BLNAR) phenotype, but were beta-lactamase producers showing low level ceftriaxone resistance (early extended spectrum beta-lactamases?) amoxycillin susceptible strains with low level ceftriaxone resistance (PBP modification?) and isolates with high-level fluoroquinolone resistance. Moraxella catarrhalis resistance to ceftriaxone erythromycin or fluoroquinolones was noted. The quantitative evolution of antibiotic resistance may reach saturation in some countries with a very high proportion of resistant strains, for example, Spain and France. Qualitatively, resistant strains may be selected that have broader or more effective mechanisms of resistance, particularly under the recently introduced pressure of more active antibiotics of the same family. In countries with modest levels of antibiotic resistance (UK, Germany, Italy), attention should be paid to the misuse of antibiotics with a propensity to select low-level resistant strains. In this respect, the relative prescribing of aminopenicillins and oral cephalosporins in the UK (a high ratio and low prevalence of Streptococcus pneumoniae) and resistance to penicillin in the USA (a low ratio and high prevalence of resistance) is of potential importance.