Alkali metal polymetaphosphate complexes of doxycycline (I) were prepd. to increase I blood levels and counteract pharmacol. disadvantages due to chelation of tetracyclines. Thus, metaphosphoric acid in CHCl3 and MeOH was treated with I to give a soln. which was treated with NaOH in MeOH. After washup I polyphosphate mono-Na complex (II) [56648-96-5], m. 184-8.degree., was obtained. Administration of II gave an av. overall increase of nearly 50% serum levels compared to I hyclate.