A group of peptide antibiotic agents, isolated from several species of Streptomyces (originally Actinomyces), which are active against gram-positive bacteria, fungi, and neoplasms. Actinomycins are chromopeptides, most containing the chromophore actinocin, and are derivatives of phenoxazine that differ in their amino acids and their sequence in the peptide chains; they form complexes with DNA and therefore inhibit RNA synthesis, primarily the ribosomal type.