Female rats were given 100 mg./120 g. body wt. of 3-(mercaptomethyl)-4-(hydroxymethyl)-5-hydroxy-6-methyl-pyridine by stomach tube, and 60 min. later were subjected to whole body x-irradiation of 300-700 r. Female mice of about 20 g. wt. were also given 3 mg. intraperitoneally 5 min. before receiving similar irradiation. Controls received irradiation only. The number of animals dying was significantly less in the treated than in the controls (P = 0.01-0.001). When 3-(mercaptomethyl) pyridine was tested in the same way it had no effect (P = 0.8-0.9). The author has no satisfactory explanation to offer.