Streptomyces refuineus, the microorganism which produces the DNA reactive antibiotic anthramycin, has been shown to possess a quite specific mechanism to survive and grow in the presence of this antibiotic. Stationary phase cells are insensitive to anthramycin since the antibiotic is prevented from entering these cells. However, cells in early log phase are inhibited by concentrations of anthramycin that are later produced by these same cells. Significantly, sibiromycin, a closely related antibiotic, is taken up by cells of S. refuineus independent of the age of the culture. Anthramycin reacts in vitro equally as well with DNA isolated from S. refuineus and other procaryotic and eucaryotic cells. When S. refuineus has reached the production phase the anthramycin is probably biosynthesized outside the cell membrane which also becomes specifically impermeable to anthramycin.