To find the cause of the mysterious disease among artificially fed infants, which has been studied in both the medical and pharmaceutical fields since late June, 1955, the Hygienic Laboratory of Okayama Prefecture started in August 24 to test M.F. 5519, which had been taken by the infants hospitalized at the hospital, based on the suspicion, voiced by the head physician of the Pediatrics Department of National Okayama Hospital on August 22, 1955, that Morinaga M.F. dried milk caused this disease. It was not evident from the preparatory Reinsch reaction test, but, using the Gutzeit method, a large amount of arsenic in this dried milk was recognized. As a result of the determination, 2.5-2.8 mg of AS203 per 100 g of M.F. 5519 were detected. Also, from 100 mg each of M.F. 5515, 5516, and 5806, obtained directly from Morinaga Milk Industry's Osaka branch office, 1.8 mg, 2.4 mg, and 2.6 mg, respectively, were detected.