The present investigation was undertaken to identify the promoting factor in the supernatant that greatly stimulates mycolic acidbiosynthetic activity localized in the cell wall fraction of B. matruchotii cells. The data obtained indicated that the factor was composed of at least two components: ATP and an undefined substance. ATP provoked activation in mycolic acid synthesis in the presence of small quantities of the supernatant whereas it had no stimulant effect in the absence of the supernatant. In view of the results of partial characterization of the promoting substance(s), it is assumed that the undefined substance is a cofactor or intermediate different from the those previously expected
