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4-Acetamido-4'isothiocyanato-stilbene-2,2'disulfonic acid (SITS) is a fluorochrome that can be used for immunofluorescence. It has an excitation maximum at 375 nm and an emission maximum at 420 nm (blue fluorescence) and thus is an ideal third fluorochrome. Conjugation should be performed under strict pH control, because the pH of the medium decreases quickly at the beginning of the conjugation process. The entire conjugation process is similar t o FITC and TRITC conjugation. The labeling should be not too high, for selecting of F/P ratios is impossible, because the sulfonic acid groups are too negatively charged for ion exchangers. We used 2 mg SITS for 100 mg protein and obtained an F/P ratio of 1.8 after DEAE fractionation at pH 7.7. Results of tests with this fluorochrome, added t o those with FITC and TRITC conjugates, show that combination of the three fluorochromes is feasible. This will increase the possibilities of imrnunofluorescence.