1. The interaction of two substances (e.g. drugs, hormones, or enzymes) having opposing effects in a system in such a way that the action of one partially or completely inhibits the effects of the other. For example, one group of anti-cancer drugs acts by antagonizing the effects of certain enzymes controlling the activities of the cancer cells. See also antagonist.

2. An interaction between two muscles, known as antagonistic muscles, in which contraction of one prevents that of the other. For example, the biceps and triceps are an antagonistic pair. See skeletal muscle.

3. An interaction between two organisms (e.g. moulds or bacteria) in which the growth of one is inhibited by the other. Compare synergism.