The International Unit of vitamin A activity was defined in 1934 as the biological response given by 0.6 pg. of pure beta carotene. Subsequently, the United States Pharmacopoeia prepared and distributed several standards consisting of cod liver oil preparations to which were assigned certain biological potencies. These preparations, however, did not prove to be suitable and their biological potency in terms of beta carotene was apparently greatly over-rated. With the isolation and availability of pure vitamin A esters, a new standard was adopted by the United States Pharmacopoeia consisting of a solution of vitamin A acetate in cottonseed oil. On the basis of the vitamin A content of this preparation, 0.3 pg. gives one unit of vitamin A activity. This new unit of vitamin A was presumed to be identical to the old unit based on 0.6 pg. of carotene. If such were the case, it would appear that the organism is able to recover only one molecule of vitamin A from one molecule of carotene.During the past few years, several publications have appeared (2, 3, 17, 20) indicating that under the proper conditions, all-trans beta carotene gives a biological response approaching that given by an equal weight of vitamin A. This agreed with the fact that biological assays on our commercial carotene products derived from carrots were higher than could be explained on the basis of their carotene content.At the present time, most of the carotene being manufactured is used in margarine where its biological activity is included as a part of the vitamin A potency. Inasmuch as the U.S.P. biological assay is the only officially accepted method for the determination of the vitamin A activity of margarine, it is obviously of great importance to know the biological activity of carotene in terms of this assay method. EXPERIMENTALWith but one exception, all of the assays reported here were on crystalline carotene products derived from carrots. Many of the samples were commercial materials which cdntained some alpha carotene along with the beta carotene.The relative amounts of the two carotenes were determined by chromatographic separation on columns of magnesium oxide (Westvaco No. 2642) : Hi-Flo Super Cel or of calcium hydroxide followed by spectrophotometric assays. The concentration of beta carotene was calculated by measurement of the absorbance a t 450 mp in hexane solution and use of an extinction coefficient (EiTJ of 2520. Alpha carotene was measured a t 446 mp using an extinction coefficient of 2720. I n addition, the absorbance ratio

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