Benzoporphyrin derivative (BPD), synthesized from protoporphyrin and involving the formation of Diels-Alder adducts, contains four components, mono and di-acid derivatives of either ring A or ring B fused porphyrins. These compounds have been isolated and tested individually as photosensitizers both in vitro and in vivo. All forms of BPD are potent photosensiters in vitro and have a strong absorption peak at about 690 nm. 3H-BPDs were tested for biodistribution in tumor bearing mice. Distribution appeared to be similar to distribution data published by others for dihematoporphyrin ethers, with the exception that levels in skin were lower for BPD. Extraction of 3H-BPD from tissue, followed by in vitro testing of extracts for photosensitizing activity, indicated that BPD is aTly degraded in vivo. When BPDs were compared to Photofrin II for causing skin photosensitivity following intravenous administration, it was found that monoacid derivatives caused photosensitivity at 3 hours but not after 24 hours. The diacid forms did not cause appreciable skin photosensitivity even at 3 hours. Photodynamic therapy of tumors in mice with BPD showed that these compounds had comparable efficacy to Photofrin II under the experimental conditions used here.