To analyse the influence of antibiotic therapy on the faecal flora of patients from general practice with complaints of a respiratory tract infection (RTI), 189 paired faecal specimens were collected, before and after completing antibiotic treatment (n=129) and symptomatic treatment (n=60). Faecal specimens were examined for the prevalence and degree of resistance to amoxycillin, apramycin, ciprofloxacin, nalidixic acid, neomycin, nitrofurantoin, oxytetracycline, sulphamethoxazole and trimethoprim. In the antibiotic-treated group a significant increase in the prevalence of resistance to amoxycillin post-treatment from 50% to 64% (PEscherichia coli isolates post-treatment had a significantly increased resistance rate to amoxycillin (15%–23%) and to neomycin (2%–6%) (P <0.05, Wilcoxon). Logistic regression analysis showed a cross resistance to neomycin and kanamycin, and for kanamycin cross-resistance to apramycin, neomycin and streptomycin occurred.