The clinical efficacies of 50 mg/kg.day miocamycin and 60 mg/kg.day amoxycillin were studied in 23 patients aged 3-11.5 years with presumed bacterial infection of the lower respiratory tract (bronchopneumonia and acute bronchitis). During the therapy, which continued for 10 days, non-specific immune function, represented by natural killer cell activity, was monitored by measurement of the rate of lysis induced on target K-562 51Cr-labelled tumour cells. The results confirmed the therapeutical efficacy of miocamycin and amoxycillin in the oral therapy of bronchopneumonia and acute bronchitis in paediatric patients. The natural killer cell activity of patients treated with miocamycin was increased on days 7 and 10 of therapy compared with baseline. This finding did not occur in patients treated with amoxycillin.