We present additional evidence that plasma from patients with deep-seated mycoses contains (1-->3)-beta-D-glucan. Digestion of such samples with endo-(1-->3)-beta-D-glucanase completely abolished the ability of the plasma to activate factor-G, a horseshoe crab coagulation enzyme that is extremely sensitive to this polysaccharide. Measurement of plasma (1-->3)-beta-D-glucan is a promising method for the diagnosis of deep-seated mycoses and for monitoring the response of these infections to antifungal therapy.