Treatment of doxycycline (I, .alpha.-6-desoxy-5-hydroxytetracycline) with HPO3 (mole ratio I-HPO3 = 1-5:1) in an inert medium (e.g., CHCl3 or MeOH), then treating the excess HPO3 with methanolic or ethanolic NaOH or KOH gave a doxycycline alkali polyphosphate complex x I.y MPO3.z HPO3 [M = Na or K, x = 1-5, y = 1-5, z = 1-5 (y+z = 6, x .ltoreq. z], the soly. of which is such that, when taken internally, it maintains a more even and higher oxycycline concn. in blood than the known doxycycline hyclate.