The fungitoxicity of mercuric chloride to Aspergillus niger was increased in the presence of D-, L-, DL-methionine, DL-ethionine, DL-S-methylcysteine or sodium methylmercaptide. The same effect was observed with methionine for two other fungi investigated: Cladosporium cucumerinum and Scopulariopsis brevicaulis. It is suggested that this effect can be ascribed to the formation of CH3SHg+ or (CH3S)2Hg, or the corresponding ethyl compounds. CH3SHgCl and (CH3S)2Hg were synthetically prepared and proved indeed far more fungitoxic than HgCl2. The hypothesis was further substantiated by the observation that A. niger rapidly converts DL-methionine into CH3SH, which undoubtedly reacts with Hg2+ to give the above mentioned methylthiomercury compounds. Chemicals/CAS: cystathionine, 56-88-2; cysteine, 4371-52-2, 52-89-1, 52-90-4; dimethyl sulfide, 75-18-3; ethionine, 13073-35-3, 67-21-0; homocysteine, 454-28-4, 6027-13-0; mercurous chloride, 10112-91-1, 7546-30-7; methionine sulfone, 1118-85-0, 7314-32-1; methionine sulfoxide, 454-41-1; methionine, 59-51-8, 63-68-3, 7005-18-7; s methylcysteine, 7728-98-5; Antifungal Agents; Cysteine, 52-90-4; Ethionine, 13073-35-3; Mercury, 7439-97-6; Methionine, 63-68-3; Sulfhydryl Compounds