1,4,6,8-Tetramethyl-2H-furo[2,3-h]quinolin-2-one [FQ] is an angelicin isoster characterized by a strong photosensitizing activity FQ shows a significant antiproliferative activity also in the dark, i.e., without UVA activation. The cytotoxic activity of FQ in the dark was detected in HeLa cells and in normal human lymphocytes; FQ showed notable antiproliferative effects, barely lower in comparison with ellipticine, used as a reference Similar results were obtained studying the FQ's capacity for forming chromosome aberrations. For both FQ and ellipticine, the chromosomal damage correlated closely with cell killing, when compared with ellipticine at the same levels of survival, FQ appeared to be much less genotoxic. Using alkaline elution we have investigated the ability of FQ to damage DNA. The formation of equivalent amounts of single-strand breaks (SSB) and DNA-protein cross-links (DPC) was observed; in addition, these lesions appeared to be located at the same sites in DNA. Experiments carried out with neutral elution demonstrated the formation of double-strand breaks (DSB). All these data are consistent with an inhibition of topoisomerase II; this hypothesis was confirmed performing an enzymatic test in vitro using topoisomerase II from Drosophila melanogaster embryos.