This study evaluates the individual and interaction effects of wood extractives, acid copper chromate (ACC), and boric acid (B) on the resistance to fungus of treated wood species. Walnut (Juglans regia L.) heartwood extractives were extracted with hot water, methanol, and ethanol solvents. Test specimens were prepared from beech sapwood (Fagus orientalis) to meet BS 838 (1961) requirements, then exposed to white-rot fungus, Trametes versicolor, for 14 weeks under laboratory conditions. Extractives of walnut heartwood contributed to increased resistance against fungus attack in the presence of B preservative only. The lowest weight loss (0.12%) occurred in the samples treated with 3.5% hot water extract and 1% boric acid, and the highest weight loss occurred in the control samples (23.7%). Results indicated that there was significant difference between the weight loss and actual retention for all treatments, but there was not any significant difference between the weight loss of treatments containing B preservative. The weight loss of samples treated with hot water extract (18.32%) was less than samples treated with methanol and ethanol extracts (21.5% and 23.1%, respectively). There was significant difference between the individual and interaction effects of wood extractives on the resistance to fungus of treated wood species. An emulsified mixture of B and walnut heartwood extractives controlled decay fungus on beech wood better than the mixture of ACC and walnut heartwood extractives, but ACC alone controlled decay fungus on beech wood better than the emulsified mixture of ACC and walnut heartwood extractives.