The fumonisins, a family of structurally related mycotoxins isolated from cultures of Fusarium moniliforme strain MRC 826 (a toxic, hepatocarcinogenic, and mutagenic isolate from maize in a high oesophageal cancer area), were characterized. Cultures of F. moniliforme grown on maize were extracted with aqueous methanol, and the bioactive extract was fractionated using macroreticular polystyrene resin (XAD-2), Sephadex LH-20, and reversed phase silica gel chromatography to isolate fumonisin A1, A2, B1, and B2. The structures of these toxins were elucidated by liquid secondary ion mass spectrometry (of diazomethane-derived tetramethyl esters) and 1H/13C nuclear magnetic resonance spectroscopy (utilizing DEPT, COSY-45, and long-range carbon-proton correlation techniques). The fumonisins are diesters of propane-1,2,3-tricarboxylic acid with either 2-acetylamino-12,16-dimethyl-3,5,10,14,15-pentahydroxyicosane (fumonisin A1) or its C-10 deoxy analogue (fumonisin A2), or 2-amino-12,16-dimethyl-3,5,10,14,15-pentahydroxyicosane (fumonisin B1) or its C-10 deoxy analogue (fumonisin B2). In all cases, the C-14 and C-15 hydroxyl groups of the icosane moiety form ester linkages with the terminal carboxyl groups of propane-1,2,3-tricarboxylic acid. Fumonisin A1 and A2 contain an acetylamino group, while B1 and B2 have a free amino group; A2 and B2 lack the C-10 hydroxyl group present in A1 and B1, respectively.