Three sesquiterpenoids that are found in the edible mushrooms Lactarius deliciosus, L. deterrimus and L. sanguifluus, have been assayed for biological activity. The compounds tested were a stearic acid ester of a sesquiterpene (I) and a sesquiterpene aldehyde (lactaroviolin, II) and alcohol (deterrol, III). The assays used were for mutagenic activity in the Ames Salmonella assay, for antimicrobial activity against bacterial fungi and algae, for cytotoxicity against Ehrlich ascitic tumour cells and L 1210 cells, and for phytotoxic activity against Lepidium sativum and Seteria italica. All three compounds showed weak mutagenic activity in the Ames assay. Two compounds (II and III) were found to have moderate cytotoxic activity and one (III) exhibited weak antibacterial activity. No compound revealed phytotoxic, algicidal or antifungal activity.