Cytotoxicities of twenty-seven fungal naphtho-gamma-pyrone derivatives to KB cells were examined. Chaetochromins A-D (1-4) and ustilaginoidins D (10) and E (11) exhibited strong effects and the structure-activity relationships are discussed. The antitumor effects in vivo of some of these compounds were examined but, due to their toxicity and to their marginal activity, development as antitumor agents was abandoned. Deoxyribonucleic acid, ribonucleic acid and protein synthesis in KB cells were equally inhibited by chaetochromin A (1), cephalochromin (5), and ustilaginoidin A (7) in parallel with the cytotoxicity of the drugs. Thus it was suggested that other still unknown mechanism(s) might induce cytotoxic lesion, resulting in the inhibition of the macromolecule syntheses.