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]