The effect of the photoproducts of indole-3-acetic acid sensitized by riboflavin on nonirradiated human HL-60 and murine NSO/2 tumor cells was studied. Severe damage with a dose-response effect was observed on both cell types. The effect was greater than that previously described for the tryptophan riboflavin photoproducts. Electron microscopy studies and flow cytometry analysis of DNA fragmentation allowed us to conclude that the photoproducts studied in this work induce cell death by an apoptotic mechanism.