Actinomycin D is one of a number of polypeptide antibiotics isolated in Waksman's laboratory.1' 2 Bacteriostatic effects, particularly on gram positive bacteria, and antitumor activity have been attributed to this compound.2 3 Kirk4 has demonstrated that the addition of actinomycin D (0.2 to 0.5 ,4M) to exponentially growing cultures of Staphylococcus aureus stops RNA synthesis immediately. This effect is rapidly followed by an inhibition of protein synthesis, and later by a partial inhibition of DNA synthesis. The action of this compound is not related directly to energy production since both respiration and glycolysis of inhibited cells are unaffected by concentrations up to 0.1 mM.4 Kirk also demonstrated that the combination of DNA and actinomycin D results in a spectral change of the latter compound. These observations suggest the formation of a complex between these two compounds since Kawamata and Imanishi5 found no interaction of actinomycin and RNA and the reaction appears to be relatively specific for DNA. Although Rauen et al.6 have reported complex formation between actinomycin and RNA, 100 times more RNA than DNA is required.Two recent reports have suggested that actinomycin acts by inhibiting the synthesis of "messenger" RNA. Nakata et al.7 have shown that reproduction of T2

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