Glucans are a, b or a and b linked glucose polymers which may play a role in environmental inhalation toxicity. The mechanism(s) by which glucans act may involve binding to a human macrophage receptor. We examined the binding of five fungal (1 3)-b-D-glucans, a barley glucan and two non-b linked glucose polymers to the human macrophage cell line, U937. Competitive binding assays employed U937 (1 x 10⁶), 15 mg of ³H-(1 3)-b-D-glucan phosphate and increasing concentrations of unlabeled polysaccharides. Non-b-linked polymers did not bind. Scleroglucan, a branched (1 3)-b-D-glucan, showed a IC₅₀= 23 nM. Other glucans showed dissociation constants of 11 to 43 mM. Barley glucan exhibited a very low affinity binding K_D>50 mM. These data indicate that the glucan receptor is specific and that it has increased affinity for certain types of glucan.