Modified H2 relaxms which act as antagonists of the relaxm receptor in cells and tissues, in particular, modified H2 relaxms comprising one or more alterations of the ammo acid sequence at positions B 13, B17 and B20 located in the receptor binding domain The antagonists retain affinity to the receptor, but do not substantially activate the receptor once bound thereto The H2 relaxm antagonists are used in compositions and methods for the treatment of cancers wherein a relaxm receptor is expressed.