Experimental evidence is provided to show that ‘classical’ C3 in the serum of patients with pigeon-fancier’s lung – in its acute, subacute or chronic form – is rapidly depleted by incubation with antigens from pigeon excreta. The most effective antigens, which are active in the nanogram range, reside among the aspecifically-precipitating ‘A’ heteropolysaccharides. With the aid of an R3 reagent, a simple laboratory test has been developed which is highly significant for pigeon-breeder’s disease (PBL). The results indicate that C3 inactivation in the serum of these patients is mediated by the – unusually labile – bypass mechanism of complement consumption, without participation of the early components. A discussion is given of the possible contributions of the various antigens in pigeon excreta to the mechanisms of pathogenesis of PBL.