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The photobiological properties of a series of pyranocoumarin derivatives having linear (xanthyletines) and angular structure (seselins) have been studied; while the linear derivative carrying the methyl geminal group, typical of the parent natural compound, appeared to be entirely inactive in all tests performed, very probably because of the steric hindrance in the dark interaction with DNA, 4 substances lacking in this group showed a marked capacity for inhibiting DNA synthesis in Ehrlich cells. In particular, 4,6-dimethyl-8-desmethylseselin proved to be about 7 times more active than 8-MOP. Practically all compounds were capable of inducing cross-links in DNA, but this feature, marked in the linear compounds, is very much reduced in the angular ones; this property appears to be clearly related to the mutagenicity in the light and with the skin phototoxicity, which are both marked in the former and low or absent in the latter. In the dark, while all compounds are non-mutagenic in the absence of metabolic activation, in the presence of microsomial enzymes pyranocoumarins become mutagens; only the xanthyletine derivative carrying a geminal methyl group at the 8 position was not activated, suggesting that the enzymes metabolize the pyranic ring.