A mutant strain of Penicillium citrinum grown in a chemically-defined production medium, yielded 145 mg compactin l(-1). The medium also facilitated spectrophotometric analysis of compactin. Addition of KH2PO4 in the production medium did not increase the compactin production, while addition of a surfactant, Tween 80, increased compactin to 175 mg l(-1). Inoculation with 10(7) spores ml(-1) and initial pH of 6.5-7 were the most suitable for compactin production.