The Collaborative International Dictionary of English v.0.48:

antimycin \an`ti*my"cin\ ([a^]n`t[i^]*m[imac]"s[i^]n), n.
   1. one of several antibiotic substances produced by several
      species of Streptomyces, active against various fungi.
      They are used only experimentally, not in medicine. The
      two best-known antimycins are antimycin A1 (C28H40N2O9)
      and antimycin A3 (C26H36N2O9). They are notable for
      their mechanism of action, interference with the proton
      pumping mechanism of fungi.
      [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

antimycin
    n 1: a crystalline antibiotic active against various fungi