Erythromycin resistance induced in Corynebacterium diphtheriae strains by subinhibitory concentrations of this antibiotic has been investigated. Six strains of C. diphtheriae highly susceptible to erythromycin were grown in the broth with 0.5 minimal inhibitory concentration (MIC). After 23-25 cultivations the susceptibility to erythromycin in the strains tested began to decrease gradually. After 46-52 consequent cultivations the strains became moderately resistant, and after 60-72 ones 5 cultures became resistant. The induced resistance to erythromycin was associated with that to oleandomycin. The strains of C. diphtheriae with induced resistance had no changes in morphology of colonies, saccharolytic and adhesive properties and toxigenicity, but their growth was slower than that of the original strains.