The ammonium sulfate-precipitated fraction from mycelia and culture-filtrates and the crude, cell-free culture filtrates from the growth medium of the fungi Chrysosporium palmorum, Eurotium rubrum, Micromucor isabellina, and Pythium aphanidermatum when aseptically added to cell suspensions of Cantharanthus roseus caused a rapid and dramatic increase in indole alkaloid biosynthesis. Up to 400 micrograms/L ajmalicine and 600 micrograms/L catharanthine were detected in C. roseus cell suspension grown in the presence of the M. isabellina fungal culture filtrate for 3 d. Untreated cells produced only trace levels of ajmalicine and catharanthine per liter of cell suspension after 15 d of culture.