The structures of the fumonisins, a family of structurally related mycotoxins isolated from cultures of Fusarium moniliforme, were elucidated by mass spectrometry and 1H and 13C n.m.r. spectroscopy as the diester of propane-1,2,3-tricarboxylic acid and either 2-acetylamino- or 2-amino-12,16-dimethyl-3,5,10,14,15-pentahydroxyicosane as well as in each case the C-10 deoxy analogue; in all cases both the C-14 and C-15 hydroxy groups are involved in ester formation with the terminal carboxy group of propane-1,2,3-tricarboxylic acid. An isolate of F. moniliforme (strain MRC 826) from maize in a high oesophageal cancer area was highly toxic to animals, inducing leukoencephalomalacia, hepatocarcinogenicity, and mutagenicity. Extraction of its cultures grown on sterilized maize with aqueous methanol, followed by fractionation using XAD-2 resin, Sephadex LH-20, and reversed-phase silica gel chromatography, isolated fumonisin A1, A2, B1, and B2. Treatment of A1/A2 with diazomethane gave tetramethyl ester derivatives, analyzed via liquid secondary ion and electron impact mass spectrometry (confirming molecular ions, empirical formulas) and NMR (DEPT, COSY-45, long-range correlations) to determine connectivity, functional groups, and fragmentation patterns. Basic hydrolysis yielded trimethyl propane-1,2,3-tricarboxylate, confirming the acid moiety. Fumonisin B1/B2 lack the N-acetyl group of A1/A2, and A2/B2 are C-10 deoxy analogues of A1/B1, as shown by mass spec and NMR comparisons.