In most of the extensive studies in man of lactic acid excretion (Feldmann and Hill, '11; Ryffel, '09-'10; Lewis et al., '25; Ldljestrand and Wilson, '25; Johnson and Edwards, '37) no distinction has been made between the optical isomers. In those instances where L-(+)- and D-(-)-lactic acid were separately estimated, optical rotation was the means of distinguishing the isomers (Craig, '46). This method is insensitive due to the low molar rotation of lactic acid. D-Lactic acid is poorly utilized in the mammalian organism (Cori and Cori, '29) although it is significantly but weakly glycogenic in the starved rat. Studies in normal and thiamine-deficient rats indicate that D-lactic acid is excreted normally by this species (van Eys et al., '62). In these studies lactic acid was measured by both a specific enzymatic technique and microbiologically, using a mutant of Lactobacillus casei, designated 280-16. This mutant was described by Camien and Dunn ('53), and was the subject of extensive studies by these investigators. It responds also to other D-α-hydroxy acids (Camien and Dunn, '53; '54) and to a lesser extent to D-1,2 alkanediols (Camien et al., '56). However, in urine D-lactic acid comprises apparently a major portion of the growth stimulating substances in urine.