It has been shown that glucan (isolated from baker's yeast) is capable of causing a decrease in reactivity of guinea pig airway smooth muscle (Jones and Nicholls, 1998). The present experiments were designed to help determine the mode of action of this (1 3)-b-D-glucan and the possible mediators involved. In particular, to ascertain if glucans have any effect on the arachidonic acid cascade. Using the immersed trachea and perfused lung models, various pharmacological tools were employed to try to modify the hypo-responsiveness seen after exposure to glucan. Antagonists of cyclo-oxygenase blocked the effect of the (1 3)-b-D-glucan on the trachea. However, in the lung, the results suggested that the 5-lipoxygenase pathway is probably of more importance. Overall, it appears that, this glucan may mediate some of its effects through the arachidonic acid cascade.