Platinum complexes of formula Ia or Ib ##STR00001## where each A, is a leaving group and may be the same or different, which is suitably halo, especially chloro, hydroxy, carboxylate or together form a bi-dentate carboxylate or sulphate, each B, which may be the same or different, is halo, hydroxy, carboxylate, carbamate or carbonate ester, Z is a substituted amine wherein the substituent sterically hinders access of the Pt atom to a DNA strand of a tumour cell, and X is NH.sub.3 or mono- or di-alkyl substituted NH.sub.3, are active against cancer cells, and appear to show a unique form of chemical and biological action.