The Collaborative International Dictionary of English v.0.48:

Antagonize \An*tag"o*nize\, v. t. [imp. & p. p. Antagonized;
   p. pr. & vb. n. Antagonozing.] [Gr. ?. See Antagonism.]
   To contend with; to oppose actively; to counteract.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Antagonize \An*tag"o*nize\, v. i.
   To act in opposition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antagonize
    v 1: provoke the hostility of; "Don't antagonize your boss"
         [syn: antagonize, antagonise]
    2: act in opposition to [syn: antagonize, antagonise,
       counteract]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "antagonize":
   aggravate, alienate, battle, be antipathetic, be inimical,
   beat against, beat up against, breast the wave, buck, buffet,
   buffet the waves, clash, close with, collide, combat, compete with,
   conflict, conflict with, confute, contend against, contest,
   contradict, contrapose, contravene, counter, counteract,
   counterattack, counterpose, countervail, counterwork, cross,
   embitter, envenom, estrange, exacerbate, fight, fight against,
   go against, go counter to, grapple with, heat up, infuriate,
   interfere with, join battle with, labor against, lock horns,
   madden, meet head-on, militate against, offer resistance, oppose,
   oppugn, provoke, reluct, reluctate, resist, rival, run against,
   run counter to, set against, set at odds, stem the tide,
   strive against, struggle against, swim upstream, take on, vie with,
   work against