Among the most useful of the broad-spectrum antibiotics is a small group of substances derived from perhydronaphthacene and called the tetracyclines. Tetracycline, 7-chlorotetracycline, and 5-hydroxytetracyclinel are used in therapy.We now wish to describe four members of a new family of compounds closely related to the previously known tetracyclines. On the basis of physical and chemical properties presented here and on the basis of degradation studies presented in the accompanying papers, it has been established that these four new compounds are 6-demethyltetracycline (I), 7-chloro-6-demethyltetracycline (II), 6-demethyl-4-epi-tetracycline (III), and 7-chloro-6-demethyl-4-epi-tetracycline (IV) .