The new 3-(4,5-dimethylthiazol-2yl)-5-(3carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt (MTS) and 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5[(phenylamino)carbonyl]-2Htetrazolium hydroxide (XTT) calorimetric assays for assessing hepatocyte density, viability and proliferation were evaluated and compared with 3-(4,5-dimethlthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and tritiated thymidine (3HTdR) incorporation. OD values of MTS or XTT, which are metabolically reduced in viable cells to a watersoluble formazan product and can be read directly, had a good correlation coefficient with hepatocyte densities in a range of 2.5-40 x lo4 cells/ml (MTS r = 0.952; XTT r = 0.902) and with hepatocyte viability (MTS r = 0.974; XTT r = 0.975). At 0, 20, 40 and 60 hr cultures, the correlation coefficients with 3HTdR for assessing hepatocyte viability and proliferation (MTS r = 0.942-0.981; XTT r = 0.953-0.992) were excellent. In contrast with MTS and XTT, MTT OD values had a poor correlation coefficient with hepatocyte densities (r = 0.672), viability 0. = 0.622) and 3HTdR incorporation (r = 0.701-0.818 at 0, 20 and 40 hour cultures).
This study shows that the MTS or XTT calorimetric assays for assessing hepatocyte density, viability and proliferation are more accurate, reliable and simpler than the widely used MTT assay. They avoid use of radioactive material as required for 3HTdR incorporation. Of the two, the MTS calorimetric assay is more sensitive than the XTT.