In the course of examining the effects on cells of polyamines, their metabolites, and polyamine analogs, it is often necessary to make some measure of cellular activity as an indicator of cell damage or cytotoxicity. One of the simplest assays utilizes 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT), a water-soluble yellow dye that is readily taken up by viable cells and reduced by the action of mitochondrial dehydrogenases ( 1 ). The reduction product is a water-insoluble blue formazan, that must then be dissolved for calorimetric measurement. Ethanol or propanol ( 2 ), acid-isopropanol (0.04 M HCl in propan-2-ol) ( 3 ), acid-isopropanol plus 10% Triton X-100 ( 4 ), mineral oil (unspecified), or dimethyl sulfoxide (DMSO) ( 5 ) have all been suggested. In the author’s hands DMSO was found to be the most satisfactory (Fig. 1 ).   Fig. 1.  Effect of solvent on formazan absorbance. EtOH, ethanol, PrOH, propanol, iPrOH, 2-propanol, iPrOH/HCl, 0.04 M HCl in 2-propanol; iPrOH/HCl/TX, 0.04 M HCl in 2-propanol plus 10% Triton X-100; DMSO, dimethyl sulfoxide (error bars show S.D., n = 5).