Alkali-metal polymetaphosphate complexes of doxycycline (I) were prepd. while not only having improved clin. activity but also increasing the clin. usefulness of I by increasing its blood-level concn. Also, blocking of I-functional groups considerably diminished chelation of Ca2+ and Mg2+ with I. Thus, to a freshly prepd. soln. of 4.74 g metaphosphoric acid in 100 mL CHCl3 and 150 mL MeOH. After keeping the clear soln. at 35.degree. for 1 h, 0.4 g NaOH dissolved in 4.4 mL MeOH was added. The product, I.cntdot.1/5 NaPO3.cntdot.1/5 (HPO3)5 [56648-96-5], was crystd. by adding 400 mL iso-PrOH.