Teleocidins produced by Actinomycetes are profoundly interesting indole alkaloids because of their tumor-promoting activity and unique structures with a nine-membered lactam ring and monoterpenoid moiety. We isolated a new metabolite named blastmycetin D (6 mg) from the mycelia (80 l) of Streptoverticillium blastmyceticum NA34-17, which produces Epstein-Barr virus early antigen-inducing indole alkaloids. The structure of blastmycetin D was established by high-resolution EIMS (molecular formula C29H45N3O3), UV (confirming indole chromophore), IR (showing lactam ring and ether linkage), and 1H NMR spectra. Chemical conversion of blastmycetin D to olivoretin A via treatment with phosphoric acid at 0°C for 2 hr indicated it is a possible precursor of olivoretin A. Isotope labeling experiments with L-methionine-methyl-13C-methyl-d3 showed that the C-23 methyl group of teleocidin B-4 originated from L-methionine, supporting the hypothesis that the C11 terpenoid moiety of teleocidins is constructed by methylation and oxidation at C-22 of a lyngbyatoxin A-type compound followed by intramolecular cyclization. These results suggest blastmycetin D is a possible precursor of teleocidins.