Biological resistance of wood treated with aldehyde cross-linking agents such as glyoxal, glutaraldehyde and dimethylol dihydroxy ethyleneurea (DMDHEU) were investigated. Sapwood blocks of Japanese cedar and Japanese beech, measuring 20 x 20 x 10 mm³ (T x R x L), were vacuum-impregnated at room temperature with 5-25% of aldehyde solutions. Blocks were kept in the solution for 1 week to gain the optimum swelling until they were sunk at the bottom, air-dried for 1 week, and cured at 120°C for 24 hours, under SO2-catalysis. After treatment, they were throughly rinsed in running water for several days to leach out the unreacted aldehyde agent. Biological resistance tests were conducted in laboratory by exposing to brown-rot fungus Tyromyces palustris, white-rot fungus Coriolus versicolor, and the two subterranean termites, Coptotermes formosanus and Reticulitermes speratus. Glutaraldehyde was most effective to eliminate the attack of Japanese cedar by all test organisms. Decay by both fungi was almost nil in the treated cedar even at the lowest 5% solution of this agent. A complete death of both termites was gained also in glutaraldehyde-treated cedar at the same concentration. DMDHEU treatment was also effective to enhance the biological resistance of Japanese cedar. Enhancement of biological resistance was recognized also in Japanese beech treated with these agents, but it was somewhat lower than in Japanese cedar. Such a difference might be related to the value of dimensional stability resulted from the treatments. Glyoxal treatment exihibited throughly a poor effect to improve the biological resistance and the dimensional stability of both wood species.