The Collaborative International Dictionary of English v.0.48:

Antithetic \An`ti*thet"ic\, Antithetical \An`ti*thet"ic*al\, a.
   [Gr. ?.]
   Pertaining to antithesis, or opposition of words and
   sentiments; containing, or of the nature of, antithesis;
   contrasted.
   [1913 Webster]

WordNet (r) 3.0 (2006):

antithetical
    adj 1: sharply contrasted in character or purpose; "practices
           entirely antithetical to her professed beliefs"; "hope is
           antithetic to despair" [syn: antithetic,
           antithetical]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "antithetical":
   adversary, adversative, adverse, adversive, alien, antagonistic,
   anti, antipathetic, antipodal, antipodean, antithetic, antonymous,
   at cross-purposes, balancing, clashing, compensating, competitive,
   con, conflicting, confronting, contradictory, contradistinct,
   contrapositive, contrarious, contrary, contrasted, converse,
   counter, counterbalancing, counterpoised, countervailing, cross,
   dead against, disaccordant, discordant, discrepant, dissentient,
   enemy, eyeball to eyeball, eyeball-to-eyeball, facing, fractious,
   hostile, inconsistent, inimical, inverse, negative, noncooperative,
   obstinate, obverse, opponent, opposed, opposing, opposite,
   oppositional, oppositive, oppugnant, overthwart, perverse, polar,
   polaric, polarized, recalcitrant, refractory, repugnant, reverse,
   rival, squared off, uncooperative, unfavorable, unfriendly,
   unpropitious