Abstract This paper reports some physico-chemical properties and cytotoxic activity of marycin, a new hematoporphyrin derivative. The data show that marycin is a new compound, different from the other porphyrins tested. This product appears to be pure by adsorption or reversed phase thin-layer chromatography and high performance liquid chromatography and has chromatographic behavior different from those of other porphyrins tested. It does not appear to link to some tested metals and has a UV-visible absorbance spectrum different from that of a hematoporphyrin methylester. Furthermore, marycin has cytotoxic activity against K-562, ZR-75, MCF-7, HT-29, LOVO, human tumor cell lines and the MRC-9 human lung embryonic cell line. The new radiometric assay was used for all cell lines. Marycin decreases the growth index measured by the radiometric assay as 14CO 2 production. The cytotoxic activity is dose-dependent. Marycin is active at low doses but the activity varies with the cell line studies. The compound has low toxicity on the normal cell line MRC-9. Marycin is very liposoluble and would be expected to have high affinity and toxicity for tumors. The compound is active without light activation. How marcyin acts is still a matter for speculation.