Crude sap, which was extracted from six Amaranthaceae plants, inhibited local lesion formation on Nicotiana glutinosa by tobacco mosaic virus(TMV) infection. Especially the remark. able inhibitory effect to TMV infection was shown on leaves of N. glutinosa precoated with the sap from Amaranthus mangostanus. The inhibitory activity of the sap from A. mangostanus was stable to storage in vitro for I day and to dilution 1/4 of the sap with distilled water. However, its activity was lost when the sap was heated at $70^{circ}C;to;100^{circ}C$ for 10 minutes. When the leaves of N. glutinosa precoated with the sap were sprayed with water, the inhibitory effect to TMV infection was maintained for 2 days. The A. mangostanus sap readjusted pH 3, pH 5, or pH 9 with 1 N HCl or 1 N NaOH did not decline the inhibitory action but the sap absorbed with $5\%;to;15\%$ charcoal completely lost their action. The protein components purified from A. mangostanus sap revealed three major bands by $5\%;to;15\%$ polyacrylamide gel electrophoresis and the top component of which showed the inhibitory action to TMV infection.