In Streptomyces peucetius var. caesius, the production of anthracyclines was suppressed either by 330 mM D-glucose or 25 mM phosphate. In addition, the anthracycline doxorubicin and the glucose analogue 2-deoxyglucose inhibited the growth of this microorganism at concentrations of 0.025 mM and 10 mM respectively. Spontaneous and induced mutants, resistant to the action of these compounds, were isolated, tested and chosen by their ability to overproduce anthracyclines. Genetic recombination between representative mutants was carried out by the protoplast fusion technique. Some recombinants carrying resistance to doxorubicin : phosphate and 2-deoxyglucose produced more than 40-fold greater levels of anthracyclines than those obtained with the parental strain. This improvement resulted in total antibiotic titres of more than 2 g/l culture medium at 6 days of fermentation.