The toxicity of ethyl-, butyl-, octyl-, benzyl-, and allyl-substituted 1-alkyl-3-methylimidazolium, alkylpyridinium, N-alkyl-N,N-dimethyl-N-(2-hydroxyethyl)ammonium (choline derivs.) and alkyl-triethylammonium salts towards HeLa cells has been studied. The relative toxicities have been detd. in the presence and absence of fetal bovine serum using an MTT assay after 24 and 48 h, and attempts to det. possible sources of the obsd. toxicity investigated using microscopy and by measuring the intracellular calcium concn., presence of reactive oxygen species and mitochondrial membrane potential in cells exposed to 1-ethyl-3-methylimidazolium tetrafluoroborate.