Ethanolic extracts of the edible mushroom Agaricus bisporus displayed a direct-acting mutagenic response in various Salmonella typhimurium strains, TA104 being clearly the most sensitive. Incorporation of an activation system derived from the liver of mice, hamsters or Aroclor 1254-induced rats failed to increase the mutagenic response. The mutagenic response of ethanolic extracts from various types of mushroom, containing different levels of agaritine (range 0.3-6.5 g/kg fresh weight), was very similar and did not correlate with the agaritine levels. Moreover, use of gamma-glutamyl transpeptidase, the enzyme catalysing the activation of agaritine, as an activation system did not enhance the mutagenicity of the mushroom ethanolic extracts. It is concluded that agaritine is not responsible for the mutagenicity of mushroom extracts.