The penultimate step in the aflatoxin biosynthetic pathway of the filamentous fungi Aspergillus flavus and A. parasiticus involves conversion of sterigmatocystin to O-methylsterigmatocystin. An S-adenosylmethionine-dependent methyltransferase that catalyzes this reaction was purified to homogeneity (> 90%) from 78-h-old mycelia of A. parasiticus SRRC 163. Purification of this soluble enzyme was carried out by five soft-gel chromatographic steps: cell debris remover treatment, QMA ACELL chromatography, hydroxylapatite-Ultrogel chromatography, DEAE-Spherodex chromatography, and Octyl Avidgel chromatography, followed by MA7Q high-performance liquid chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the protein peak from this step on silver staining identified a single band of approximately 40 kDa. This purified protein was distinct from the dimeric 168-kDa methyltransferase purified from the same fungal strain under identical growth conditions (D. Bhatnagar, A. H. J. Ullah, and T. E. Cleveland, Prep. Biochem. 18:321-349, 1988). The chromatographic behavior and N-terminal sequence of the 40-kDa enzyme were also distinct from those of the 168-kDa methyltransferase. The molar extinction coefficient of the 40-kDa enzyme at 278 nm was estimated to be 4.7 x 10(4) M-1 cm-1 in 50 mM potassium phosphate buffer (pH 7.5).