Abstract The highly dissimilar properties and toxic effects of Fusarium sporotrichioides and F. poae and of Aspergillus flavus are briefly described and discussed. F. sporotrichioides and F. poae have been shown to be the principal causes of the fatal human toxicoses caused in 1942–1944 in some districts of the U.S.S.R. Affected were people who consumed grains of wheat, barley, millet and oat that had overwintered under snow cover. The disease has been termed Alimentary Toxic Aleukia (ATA). The structure and properties of the toxins have been determined: ‘sporofusarin’, produced by F. sporotrichioides, and ‘poaefusarin’, produced by F. poae, are both sterol compounds that form at temperatures considerably below 0°C (−2°C to −15°C) and liquefy at 246–248°C and 148–150°C, respectively. Both these toxins, when fed to cats, produced the same fatal ATA syndrome that was produced by the fungal isolates. The symptoms, as in man, were necrosis of the digestive tract and of other internal organs, destruction of bone marrow, and extensive changes in the adrenal glands. On the skin of rabbits and calves application of the toxins caused necrosis and other severe reactions, such as haemorrhage and leucocytorrhea with or without oedema. The aflatoxin produced by A. flavus, especially on groundnut kernels, has been extensively studied in recent years. In feeding tests in Israel, ducklings succumbed after 5–7 days, turkey chicks after 10–14 days, and rabbits after about 40 days. Histological examination showed that aflatoxin destroys parenchyma of the liver and causes proliferation of tubules in the gall bladder. On rabbits, histo-pathological changes were also induced in the spleen, kidneys, intestines and adrenal glands. Of the known components of aflatoxin, the one termed B (B 1 and B 2) proved highly toxic. About 90 per cent of 1626 isolates of A. flavus from groundnut kernels and the soils they grew in, showed toxic properties (1·7 per cent B 1; 79·5 per cent B 1 + B 2; 8·4 per cent B 1 + B 2 + G 1 + G 2). Optimal temperature for aflatoxin production in vitro was 24°C.