Agaricus brasiliensis (A. blazei) is a new cultivated medicinal and edible mushroom, which is already popular in Brazil, Japan and China. Because of its great potential as a gourmet food, it has also drawn the attention of mushroom growers in the USA and in Europe. Since some related edible mushrooms are known to contain potentially toxic substances, notably cadmium and agaritine, cultivated A. brasiliensis from the USA, Brazil and China were investigated, not only for these toxicants, but also for other trace elements, secondary metabolites, radionuclides and pesticide residues. For comparison, we also analysed the principal cultivated and wild-growing mushrooms offered for sale on the Swiss markets. The Padi straw mushroom, Volvariella volvacea, was included in the investigation, because it is widely consumed in Asia. Agaricus brasiliensis, regardless of its origin, was found to contain the same concentrations of agaritine, urea and free tryptophan as those measured in the White cultivated mushroom, A. bisporus. Pesticide residues and radionuclides were at or below the limit of determination in A. brasiliensis in Brazil and the USA, whereas a sample from China contained a little BHC and deltamethrin. Other mushrooms also had negligible levels (if any) of these contaminants. Caesium 137 was readily detected in wild-growing mushrooms from Europe, but the levels found were well below the legal limit. Among the trace elements studied, excessive cadmium in A. brasiliensis may be a potential problem. Still, cadmium concentrations in cultivated mushrooms from Brazil and the USA complied with European legislation, whereas a sample from China did not. However, the Cd content of Pleurotus and Shiitake [Lentinula edodes], two widely consumed mushrooms, also occasionally exceeds the EEC limit, which may have been set too low. This matter and the significance of other heavy metals, arsenic and selenium are briefly discussed. Interestingly, there are important differences between levels for various trace elements in mycelium and mushrooms. It was noted that copper deficiency in the growing substrate might affect the pigmentation of the carpophores.