Over a period of 13 years 740 samples of various animal feedstuffs were analysed for the presence of aflatoxin B1. Nearly 500 were suspected of causing disease in farm animals but aflatoxin B1 could be detected in only 13.6 per cent of them. Groundnut meal samples nearly always contained the toxin but compounded feeds were contaminated to a varying extent. In particular, none could be detected in 62 poultry feeds. Where mycotoxicosis was suspected in dairy cattle, aflatoxin B1 was present at levels of more than 30 micrograms per kg in about 27 per cent of associated feed samples. In other dairy feeds (178 samples) this level of contamination was present in only about 9 per cent of samples. Concentrations of aflatoxin B1 likely to produce toxic effects (100 micrograms per kg and above) were present in about 4 per cent of suspect dairy concentrates but in 1 per cent or less of others. Contamination of dairy concentrates with aflatoxin B1 is usually at such a level that cows probably excrete the toxic metabolite aflatoxin M1 in milk at very low concentrations of 0.1 micrograms per litre or less.