The antimicrobial and cytotoxic properties of a series of 9,10-dihydrophenanthrenes structurally related to juncusol (1a), a postulated phytoalexin with confirmed cytotoxic properties, are detailed. Two simple 9,10-dihydrophenanthrenes, 2,7-dihydroxy-3,8-dimethyl-9,10-dihydrophenanthrene (2h, desvinyljuncusol) and 2-hydroxy-3-methyl-9,10-dihydrophenanthrene (3h), were found to possess in vitro antimicrobial activity comparable with that of the natural product. Two 9,10-dihydrophenanthrenes substituted with quaternary ammonium salts, 2d and 3d, each containing a reactive benzylic dimethyl[(phenylthio)methyl]ammonio group, were found to be 10-20 times more potent than juncusol (1a). Confirmed in vitro cytotoxic activity that parallels antimicrobial activity was found for juncusol (1a), desvinyljuncusol (2h), 2-hydroxy-3-methyl-9,10-dihydrophenanthrene (3h), and the quaternary dimethyl[(phenylthio)methyl]ammonium salts 2d and 3d in a human lymphoblastic leukemia cell culture (CCRF-CEM, IC50 = nt, 9.3, nt, 0.9 and 1.4 microgram/mL, respectively), B-16 mouse melanoma cell culture (IC50 = 12.5, 17.5, 27.7, 0.3, and 0.5 microgram/mL, respectively), and L-1210 mouse lymphocytic leukemia cell culture (IC50 = 13.8, 10.2, 24.5, 1.3, and 3.7 micrograms/mL, respectively). The comparable potency and spectrum of activity of juncusol (1a), desvinyljuncusol (2h), and 2-hydroxy-3-methyl-9,10-dihydrophenanthrene (3h) suggest that the agents are acting as simple phenols in exerting their antimicrobial and cytotoxic effects.