New polyisoprenepolyols (hypsiziprenol AA and BA) were isolated from an edible mushroom (Hypsizigus marmoreus). These polyols occur as a mixture of homologous polyisoprene derivatives with 40-70 carbon atoms. Analyses by FAB/MS in the positive and negative ion modes are complementary with each other in that the former provides information on the number of hydroxy groups present while the latter specifies the isoprenoid sequence, and thus become a powerful tool for analyzing the structures of polyisoprenepolyols. No polyisoprenepolyols obtained here were found to have antitumor activity on NCI-H292 and EL-4 cell lines.