A water sol. complex of .alpha.-6-deoxy-5-hydroxytetracycline (I) and Na tetrametaphosphate has good bioavailability, broad spectrum antibacterial properties, excellent stability in gastric juice as well as in cryst. form and does not the disadvantages of I. However, the major advantage of using this complex is its decreased interference in the action in human serum of natural bactericides. The complex was prepd. by the treatment of freshly prepd. HPO3 with I and later with NaOH in MeOH. After addn. of iso-PrOH, the cryst. compd. was filtered, washed and dried. The increased bioavailability of the complex was demonstrated in humans after 100 and 200 mg capsule administration.