We have noted that dose-dependent tumour growth inhibitory activity of Buna-shimeji mushroom, Hypsizigus marmoreus, against murine Sarcoma-180 (S-180) was observed following the administration of 5% and 10% powdered fruiting body-mixed feed (dose of fruiting body, ca. 16 mg/kg per day). Moreover, we have found that ethyl acetate extract from powered fruiting body also exhibited tumour-growth inhibitory activity. Approximately 3 g of the active substance (acetone-eluting fraction) was obtained from 250 g of powdered fruiting body by silica gel fractionation. Following the administration of the acetone fraction (ca. 250 mg/kg per day), equivalent to the dose present in 10% powdered fruiting body-mixed feed, the tumour volume of solid S-180 was 41.5% compared to that of control mice that were fed only CE-2. The acetone fraction also exhibited similar inhibitory activity against syngeneic IMC carcinoma. The main active substance of the acetone fraction was subsequently identified as the tandem-type polyterpene (C₄₅H₈₆O₇) by NMR and MS analysis. In addition to the murine tumour cells S-180 and IMC, we have found that this terpene fraction exhibits cytotoxicity against various human tumour cells, including U937, MKN45 and HL-60. Moreover, the purified active substance (C₄₅H₈₆O₇) was observed to induce apoptosis in HL-60 cells, and it was suggested that this activity might also explain the tumour-growth inhibitory activity.