Assays based on a biological response (bioassays) offer the possibility of screening foodstuffs for both known and uncharacterised mycotoxins. The sensitivity of several bioassays, namely brine shrimp, rat liver cells, baby hamster kidney cells, Bacillus megaterium, B. stearothermophilus Tetrahymena pyriformis and pea seedlings, to mycotoxin standards was established. Based on these results, the bacterial assays were found to be relatively insensitive to the majority of mycotoxins tested. A biological screen consisting of the above bioassays (excluding the bacterial assays) was capable of detecting twelve mycotoxins. This screen was applied to the testing of moulds isolated from mould-spoiled foods, identified, and tested in a ratio corresponding to their percentage occurrence. They included species of Aspergillus, Penicillium, Cladosporium, Rhizopus, Mucor, Alternaria and Wallemia. Approximately 60% of the moulds, when grown in culture media, caused a toxic effect in three or more of the bioassays. Some of the moulds caused enhanced toxicity when grown on a foodstuff; other extracts from mould-inoculated foods were found to be non-toxic to the bioassays. It was of interest to note the toxicity to bioassays caused by moulds such as Mucor and Wallemia. These are not well-recognised mycotoxin producers. A further study was made on toxin production by Wallemia. A scheme of chemical purification, involving TLC and HPLC, linked with bioassay testing, was used to isolate the toxic compound. The toxin, to be named walleminol A, has a molecular weight of 236 and probable composition of C15-H24O2. The minimum inhibitory dose of the toxin to bioassays was approximately 50 mug/ml. Toxicity of ochratoxin A to cell lines was not enhanced by the inclusion of microsomal enzymes. The acute toxicity of aflatoxin B1 and sterigmatocystin, was, however, greatly enhanced by the microsomal enzymes. Aflatoxin B1 and sterigmatocystin were metabolised to form the more polar metabolites. The effects of these toxins on cells was also examined by flow cytometry. This demonstrated that aflatoxin B1 and sterigmatocystin had no effect on the cell cycle unless activated by microsomal enzymes. The activated toxins inhibited DNA synthesis and showed that apparently surviving cells died on subculture.