AbstractPorphyrins are a group of organic compounds involved in a wide spectrum of fundamental biological processes. Non-metallic, naturally occuring and synthetic porphyrin derivatives may produce cytotoxic effects in malignant or normal tissues exposed to visible light. Supra-clinical doses of the photosensitizing porphyrin, Photofrin®are hematostimulatory when administered to normal and immunosuppressed inbred mice. To determine if a non-photosensitizing metalloporphyrin has similar hematostimulatory activity, we have synthesized iron (III) hematoporphyrin chloride (FeHp) and administered it to sub-lethally irradiated mice. FeHp (10 mgAg) given 1 and 4 days or 1,4 and 7 days following sub-lethal (7 Gy) whole body irradiation significantly increased spleen colony forming units of progenitor cells of the granulocyte-macrophage lineage (CFU-GM) 14 days post-irradiation, relative to irradiated controls. In addition, total splenocyte numbers were significantly increased 17 days post-irradiation in mice that had received FeHp 1 and 4 days post-irradiation. When FeHp was given 24 hours prior to irradiation and again 48 hours or 48 and 96 hours post-irradiation, significant increases in splenic CFU-GM and spleen cell numbers, relative to control mice, were observed 15 days post-irradiation. A non-metallic photosensitising monomeric fraction of Photofrin®, deuteroporphyrin IX, 2,4 (4,2) hydroxyethyl vinyl (HVD) was compared to Photofrin®for its ability to influence the hematopoietic recovery of irradiated mice. Only Photofrin®but not HVD given in 3 doses (10 mg/Ag) 1, 4 and 7 days following irradiation (4.8 Gy) significantly enhanced the recovery of spleen cellularity and splenic CFU-GM. In addition, Photofrin®significantly increased bone marrow CFU-GM 7 and 10 days following the sub-lethal dose ofγ-radiation. The mechanism by which certain porphyrins augment hematopoiesis in the mouse is unknown. However, the identification of FeHp as a non-photosensitising monomeric porphyrin with hematostimulatory activityin vivo, indicates that further study of metalloporphyrins is warranted and may reveal their clinical potential within the context of therapeutically-induced immunosuppression.