Attempts to characterize some chemical properties of the picodnaviruses RV, H-l and X 14 hemagglutinins are performed using several enzymes and reagents. The results indicate the proteinaceous nature of the viral receptors since no lipidic or glycidic moiety could be shown. Although the viral receptors proved to be resistant to proteolytic enzymes such as trypsin, chymotrypsin and papain; glutaraldehyde and p-chloromercuribenzoate fully suppressed the hemagglutinating activity of the tested picodnaviruses. The inhibitory action of p-chloromercuribenzoate was reversed by 2-mercaptoethanol in the case of RV and X 14 viruses, but not in that of H-1 virus. On the whole, the chemical properties of the picodnavirus hemagglutinins appear to be different from those of other hemagglutinating viruses. However, some similarities with the hemagglutinins of entero-, and adenoviruses are evident.