SummaryThe methanol extract factor required by the mink has been found to be soluble in 90% phenol and insoluble in acetone, n-butanol and 3/4 saturated ammonium sulfate.Fish solubles contain this factor, but dried distillers solubles, dried whey, pork spleen, and hog intestinal mucosa were found to be inactive.The relationship between the methanol extract factor and vitamin B12 is discussed.The original observation that the mink requires a methanol insoluble factor as well as a soluble factor has been confirmed by the data presented.We wish to acknowledge our indebtedness to Merck and Co., Rahway, N. J., for the crystalline vitamins; to Lederle Laboratories, Pearl River, N. Y., for the pteroylglutamic acid.