An overnight culture of S. cerevisiae was diluted 1:20 into 250 mL of fresh media and incubated for 3 h. The culture was divided into 40 mL aliquots and dosed with various concentrations of DCOIT or with other isothiazolones. After a 30 min incubation, viable counts were determined by the most probable number (MPN) method in media containing 500 mg kg sodium thioglycollate. 40 mL of the sample was centrifuged at 5 000 revolutions per minute for 5 min and resuspended in ice-cold media supplemented with 500 mg kg sodium thioglycollate. The samples were washed twice by centrifugation, each time being resuspended in 10 mL of ice-cold suppiemented media. After the final wash, cell pellets were resuspended in 1 mL of ice-cold 10% trichloroacetic acid and incubated on ice for 30 min. At this time, the samples were centrifuged at 14 000 revolutions per minute in a microcentrifuge for 2 min. The supernatant was decanted and saved. The pellet was resuspended in 1 mL of 0.4 M Tris-EDTA (pH 8.9). Samples (200 HL) of both the acid soluble extract and insoluble material were added to 1 mL of Tris-EDTA buffer, then 20 HL of dithiobisnitrobenzoic acid (DTNB)/methanol (99 mg/25 mL solution was added. After 5 min incubation of this mixture, the absorbance at 412 nm was measured. Thiol concentrations in the samples were determined from a linear equation derived from a standard curve of glutathione spiked into both the acid soluble and insoluble material samples. Acid soluble and insoluble thiols were determined using Ellman's reagent (dithiobisni-trobenzoic acid, DTNB) to react with available thiol.