Desoxyribonucleic acids were isolated from tissues of four mammalian genera. Ox thymus and liver, sheep thymus and liver, pig thymus, liver, spleen, thyroid, and human thymus and liver (including carcinomatous tissue) served as the sources of a total of 38 preparations. The contents in individual purines and pyrimidines were determined quantitatively and compared. The significance of the differences in composition, most marked when bovine and human preparations were compared, and a number of regularities are discussed. All nucleic acids appeared to yield the same sugar constituent, tentatively identified as 2-desoxyribose.We compare here the composition of the desoxypentose nucleic acids isolated, in a highly polymerized form, from different tissues of ox, sheep, pig and man.Altogether, 38 preparations were analyzed in 85 hydrolysis experiments. The number of individual determinations of the nitrogenous constituents performed with each hydrolysate averaged 18. ful significance, when analyzed statistically; but the probability of identity becomes very small, when

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