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Photosensitization processes provoked by furocoumarins on various biological systems seem to be in connection with the photoreactions that these substances give both in vitro and in vivo with pyrimidine bases of nucleic acids; in particular linear furocoumarins (psoralen) photoreact with native DNA giving both monofunctional and bifunctional additions (forming in this last case inter-strand cross-linkings) while angular furocoumarin (angelicin) can give only monofunctional additions. Previous studies on the possible recovery of this damage to DNA provoked both by linear and angular furocoumarins demonstrated that no repair underwent either by means of photosplitting experiments or through photoreactivation processes. In this paper are reported direct results indicating that the photodamage to DNA is repairable through post-irradiation dark recovery both operating on microbial cultures and on guinea-pig skin. In both biological systems monofunctional additions appear much more easily repairable than bifunctional additions; in any case bifunctional additions (which produce inter-strand cross-linkings) clearly appear to be repairable. [ABSTRACT FROM AUTHOR]