1. o-Phenylenediamine has been found to stimulate the synthesis of compounds with vitamin B12 activity by S. olivaceus in both the synthetic and the nonsynthetic media. 2. Bioautographic determinations have shown that the incorporation of OPD or benziminazole into a synthetic medium causes the formation of a new component having a lower Rf value than vitamin B12, while other compounds tested promote the formation of vitamin B12. 3. This has been confirmed by inhibition studies, and the inhibition index has been found to be about 160. 4. Implications of these results in relation to the biosynthetic formation of vitamin B12 and its analog have been discussed. © 1956, All rights reserved.