The methanolic extract from the tubers of Gymnadenia conopsea showed an antiallergic effect on ear passive cutaneous anaphylaxis reactions in mice. From the methanolic extract, three new dihydrophenanthrenes, gymconopins A ( 1), B ( 2), and C ( 3), and a new dihydrostilbene, gymconopin D ( 4), were isolated together with 10 known phenanthrene and stilbene constituents. The structures of the new compounds were determined on the basis of physicochemical evidence. Next, the inhibitory effects of the principal constituents on the release of beta-hexosaminidase, as a marker of degranulation, in RBL-2H3 cells were examined and five phenanthrenes, gymconopin B ( 2), 4-methoxy-9,10-dihydrophenanthrene-2,7-diol ( 6), 1-(4-hydroxybenzyl)-4-methoxyphenanthrene-2,7-diol ( 7), 1-(4-hydroxybenzyl)-4-methoxy-9,10-dihydrophenanthrene-2,7-diol ( 8), and blestriarene A ( 9), and six dihydrostilbenes, gymconopin D ( 4), batatacin III ( 10), 3'- O-methylbatatacin III ( 11), 3,3'-dihydroxy-2-(4-hydroxybenzyl)-5-methoxybibenzyl ( 12), 3',5-dihydroxy-2-(4-hydroxybenzyl)-3-methoxybibenzyl ( 13), and 3,3'-dihydroxy-2,6-bis(4-hydroxybenzyl)-5-methoxybibenzyl ( 14) were found to inhibit the antigen-induced degranulation by 65.5 to 99.4 % at 100 microM in RBL-2H3 cells.