It has been shown that the mushroom L. necator, which until recently was regarded in Finland and some other countries as a first class edible mushroom contains one of the strongest mutagenic compounds found in nature. This compound, called necatorin, has been tentatively identified as 7-hydroxycoumaro[5,6-c]cinnolines. The preliminary characterization of its mutagenic properties revealed it to be comparable to aflatoxin B1 (6000 revertants per µg in the Ames Salmonella assay with test strain TA 100). Although no positive response was observed in the hostmediated assay or in the micronucleus test with the pure compound, necatorin has been shown to give a positive response in the DNA repair test with mammalian cells. The compound has been found in L. necator in concentrations ranging from 3 to 20 mg/kg, and has been shown to be stable. No loss was observed in samples stored in the dark for 6 days under conditions ranging from 0.5 M hydrochloric acid to 0.5 M sodium hydroxide solution. The compound was however susceptible to decomposition by light, especially at high pH values. The boiling of necatorin in 0.5 M sulphuric acid or in 0.5 M sodium hydroxide solution for 1 h had very little effect. The published isolation method is rather laborious, requiring preparative high-performance liquid chromatography (HPLC) after extraction of mushroom samples overnight. An even more important deficiency of the method was its low and variable yield. The results of quantitative analysis performed previously, the chemical identity of the compound and its properties provide a basis for both an improved HPLC method and an improved preparative method. The former is necessary for a study of the occurrence and distribution of necatorin and the latter is required to provide material for further evaluation of the toxicological and possible carcinogenic properties.