The Collaborative International Dictionary of English v.0.48:

actinomycin \actinomycin\ n. [From actinomyces, the genus of the
   organism in which they were first found.]
   1. any of various red antibiotics isolated from soil bacteria
      with a three-ring heterocyclic nucleus with an attached
      peptide chain.

   Note: Actinomycin D is the most well-known and has been used
         to treat certain tumors. They act by binding to DNA and
         inhibiting the transcription of RNA. Their binding to
         DNA has been much studied.

   Syn: actinomycin actinomycin C actinomycin F1
        actinomycin D
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

actinomycin
    n 1: any of various red antibiotics isolated from soil bacteria