The actinomycins, particularly actinomycin-C, -D, and -K are effective in inhibiting the growth of some experimental animal as well as human tumours in extremely small doses in comparison with other antitumour drugs so far known.In view of the fact that the chemical structures of these antitumourous antibiotics possess the same chromophore grouping but differ in the polypeptido-lactone-ring moiety with respect to the kinds and number of amino-acids and the sequence of their arrangement, it is suspected that the antitumour action of these compounds may arise from the chromophore grouping in common, whereas the peptido moiety functions as the carrier of the active group.