The natural durability of western red cedar cannot be entirely explained by the presence of the thujaplicins – the only extractives identified with significant direct fungal toxicity. However, other extractives may contribute to the natural durability of western red cedar through interactions with the thujaplicins. Plicatic acid is the most abundant extractive in western red cedar and it is also a strong radical scavenger and metal chelator. To identify any interactions between plicatic acid and beta-thujaplicin, the effect of plicatic acid on the inhibition of fungal growth by beta-thujaplicin was determined using a micro-bioassay. Increased fungal growth was observed in some samples treated with plicatic acid. However, in most samples plicatic acid did not affect the ability of beta-thujaplicin to inhibit the growth of decay fungi in this agar-based bioassay.