The Collaborative International Dictionary of English v.0.48:

antagonism \an*tag"o*nism\ ([a^]n*t[a^]g"[-o]*n[i^]z'm), n. [Gr.
   'antagw`nisma, fr. 'antagwni`zesqai to struggle against;
   'anti` against + 'agwni`zesqai to contend or struggle, 'agw`n
   contest: cf. F. antagonisme. See Agony.]
   Opposition of action; counteraction or contrariety of things
   or principles.
   [1913 Webster]

   Note: We speak of antagonism between two things, to or
         against a thing, and sometimes with a thing.
         [1913 Webster]

WordNet (r) 3.0 (2006):

antagonism
    n 1: a state of deep-seated ill-will [syn: hostility,
         enmity, antagonism]
    2: the relation between opposing principles or forces or
       factors; "the inherent antagonism of capitalism and
       socialism"
    3: an actively expressed feeling of dislike and hostility
    4: (biochemistry) interference in or inhibition of the
       physiological action of a chemical substance by another
       having a similar structure

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "antagonism":
   abhorrence, abomination, aggression, aggressiveness, allergy,
   animosity, animus, annulling, antipathy, antithesis, argumentation,
   aversion, backlash, bad blood, bellicism, bellicosity,
   belligerence, belligerency, chauvinism, clash, clashing,
   cold sweat, collision, combativeness, competition, con, conflict,
   confrontation, confutation, contention, contentiousness,
   contradiction, contradistinction, contraindication, contraposition,
   contrariety, contrariness, contrast, controversy, counteraction,
   counterposition, counterworking, crankiness, creeping flesh,
   cross-purposes, crotchetiness, despitefulness, difference,
   disaccord, disaccordance, disagreement, discord, discordance,
   discordancy, discrepancy, disgust, disharmony, disparity,
   dissension, dissent, dissidence, dissonance, disunion, disunity,
   divergence, diversity, enmity, faction, ferocity, fierceness,
   fight, fractiousness, friction, hate, hatred, horror, hostility,
   inaccordance, incongruity, inconsistency, inequality, inimicalness,
   interference, jarring, jingoism, kick, loathing, malevolence,
   malice, malignity, martialism, militancy, militarism,
   mortal horror, nausea, negation, negativeness, nonconformity,
   noncooperation, nullification, obstinacy, oppositeness, opposition,
   opposure, oppugnance, oppugnancy, perverseness, perversity,
   polarity, pugnaciousness, pugnacity, quarrelsomeness, rancor,
   reaction, recalcitrance, recoil, refractoriness, renitency,
   repercussion, repugnance, repulsion, resistance, revolt, rivalry,
   saber rattling, showdown, shuddering, spite, spitefulness, strife,
   swimming upstream, truculence, uncooperativeness, unfriendliness,
   unharmoniousness, unpeacefulness, variance, vying, warmongering,
   warpath, withstanding