Actinomycin was first isolated by Waksman and Woodruff (1940) from a culture of Streptomyces antibioticus. It was found to pose marked antimicrobial properties against gram positive bacteria, and to a lesser extent is active upon gram negative bacteria and fungi. It was later shown to possess selective cytostatic effects upon mammalian tissues (Robinson and Waksman, 1942; Hackmann, 1954). Subsequent studies in various laboratories revealed that actinomycin or actinomycin-like substances are produced by a large number of Streptomyces species. The various forms of actinomycin have been designated as A, B, C, D, I, J, and X (Waksman, 1954). These actinomycins, while readily crystallized, are generally not chromatographically homogeneous substances. In more recent investigations it has been shown that they are mixtures of closely related polypeptides linked to a chromophoric quinonoid moiety (Brockniann, 1954) apparently identical for all the actinomycins (Brockmann and Muxfeldt, 1955). Physical and chemical differences observed among the various actinomycin complexes are considered to be due to variations in the number, arrangement, and kinds of amino acids present in the peptide. For example, actinomycin C can be distinguished from actinomycins A, B, D, I, and X through the presence of D-alloisoleucine in the peptide part of certain of its components. All actinomycin complexes thus far analyzed possess the amino acids sarcosine, D-valine, L-proline, L-threonine, and N-methylvaline. Recently a number of crystalline and chromatographically homogeneous components were separated from the actinomycin complexes, A, B, and D (Roussos and Vining, 1956). Comparison of their chemical and physical properties revealed that these components represented only three distinct entities (