The Collaborative International Dictionary of English v.0.48:

Antagonistic \An*tag`o*nis"tic\, Antagonistical
\An*tag`o*nis"tic*al\, a.
   Opposing in combat, combating; contending or acting against;
   as, antagonistic forces. -- An*tag`o*nis"tic*al*ly, adv.
   [1913 Webster]

         They were distinct, adverse, even antagonistic.
                                                  --Milman.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antagonistic
    adj 1: indicating opposition or resistance [syn: antagonistic,
           counter]
    2: characterized by antagonism or antipathy; "slaves
       antagonistic to their masters"; "antipathetic factions within
       the party" [syn: antagonistic, antipathetic,
       antipathetical]
    3: arousing animosity or hostility; "his antagonistic
       brusqueness"; "Europe was antagonistic to the Unites States"
       [ant: conciliative, conciliatory]
    4: used especially of drugs or muscles that counteract or
       neutralize each other's effect [syn: antagonistic,
       incompatible] [ant: interactive, synergistic]
    5: incapable of harmonious association

Moby Thesaurus II by Grady Ward, 1.0:

181 Moby Thesaurus words for "antagonistic":
   acrid, adversary, adversative, adverse, adversive, aggressive,
   alien, anti, antipathetic, antithetic, antonymous,
   at cross-purposes, at loggerheads, at odds, at variance, at war,
   averse, balancing, battling, bellicose, belligerent, bitter,
   bloodthirsty, bloody, bloody-minded, breakaway, caustic,
   chauvinist, chauvinistic, clashing, colliding, combative,
   compensating, competitive, con, conflicting, confronting,
   contentious, contradicting, contradictory, contradistinct,
   contrapositive, contrarious, contrary, contrasted, converse,
   counter, counteractant, counteracting, counteractive,
   counterbalancing, counterpoised, countervailing, counterworking,
   cranky, cross, crotchety, dead against, despiteful, detrimental,
   differing, difficult, disaccordant, disagreeable, disagreeing,
   discordant, discrepant, disharmonious, disinclined,
   disproportionate, dissentient, dissident, dissonant, divergent,
   enemy, eyeball to eyeball, ferocious, fierce, fighting, fractious,
   full of fight, full of hate, grating, hard, harmful, hateful,
   hawkish, hostile, immiscible, in opposition, inaccordant,
   incompatible, inconsistent, inconsonant, indisposed, inharmonious,
   inimical, inverse, jangling, jarring, jingo, jingoish, jingoist,
   jingoistic, malevolent, malicious, malignant, martial, militant,
   militaristic, military, miserable, negative, nonconformist,
   noncooperative, not easy, obstinate, obverse, offensive, opponent,
   opposed, opposing, opposite, oppositional, oppositive, oppugnant,
   out of accord, out of whack, overthwart, perverse, pugnacious,
   quarrelsome, rancorous, reactionary, reactive, recalcitrant,
   refractory, renitent, repugnant, resistant, reverse, revolutionary,
   rigorous, rival, saber-rattling, sanguinary, sanguineous, savage,
   scrappy, set against, sinister, soldierlike, soldierly, sore,
   spiteful, squared off, stressful, trigger-happy, troublesome,
   troublous, truculent, trying, uncongenial, uncooperative,
   unfavorable, unfriendly, unharmonious, unpacific, unpeaceable,
   unpeaceful, unpropitious, untoward, unwilling, variant, venomous,
   virulent, vitriolic, warlike, warmongering, warring, wretched