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]

The Collaborative International Dictionary of English v.0.48:

actinomycin D \actinomycin D\ n. [From actinomyces, the genus of
   the organism in which they were first found.]
   The most well-known of the actinomycins (C62H86N12O16), a
   class of antibiotics which act by binding to DNA and
   inhibiting synthesis of RNA; they act agains gram-positive
   bacteria and many eukaryotic organisma. Actinomycin D has
   been used in human medicine to treat certain tumors.

   Syn: dactinomycin, actinomycin IV, Cosmegen[trade name],
        actinomycin C1, actinomycin I1
        [PJC]