Doxycycline and alkali hydroxides were added to freshly prepd. metaphosphoric acid to give the following complexes (C22H24N2O8)n.(MPO3)m.(HPO3)p (M, n, m, and p given): Na, 5, 1, 5; Na, 4, 2, 4; K, 5, 1, 5. The polymetaphosphate complexes prepd. are useful as bactericides.