Mosmann's method for measuring the number of viable cells with a tetrazolium salt, 3-(4,5-dimethylthiazol-2-yl)-2,5-di-phenyltetrazolium bromide (MTT), was modified to make it possible to measure a large number of interleukin 2 (IL-2) samples at one time with less labor and more accuracy. Each step of the method was examined in detail and modified (the modified MTT method). An IL-2-dependent mouse natural killer cell line, NKC3, was used as an indicator cell line. The incubation period before adding MTT was reduced to 24 h, A solution of 10% sodium dodecyl sulfate-0.01 N HCl was used to dissolve the MTT formazan produced. We have compared the values obtained by the modified MTT method and the conventional [3H]thymidine method (3H-TdR method), and confirmed that the estimates of IL-2 content were almost equal. The variation of IL-2 content measured by both methods was within 5% in terms of the standard error.