The chromophore of the antitumor chromoprotein largomycin FII is a mixture of components belonging to the pluramycin class of antitumor antibiotics. Against most organisms tested, component 4 exhibited activity equal to or greater than the major chromophore components pluramycin A and deacetylpluramycin A. Data obtained from UV, IR, ¹H and ¹³C NMR, and from fast atom bombardment mass spectrometry were used to determine the structure of component 4 as epoxykidamycin, a new member of the pluramycin class.