Stc. faecalis 10CL was used for the microbiological assay of α-lipoic acid. The acid produced was measured titrimetrically after incubation for 72 hours. The concentration of DL-α-lipoic acid ranging from 0.25 to 4.0 mμg per ml was suitable for the assay. Lipoic acid activity of urine was found to be 30.9±10.8 mμg/ml in normal human beings. Total lipoic acid activity of rat liver hydrolyzed with 6NH_2SO_4 was 2.10±0.008μg/g.
