Pokeweed antiviral protein (PAP) inactivates both eukaryotic and prokaryotic ribosomes via a specific depurination of rRNA. The sensitivity of pokeweed ribosomes to PAP implies the existence of a mechanism to protect the plant. Using monoclonal antibodies specific to PAP, a protein complex (PAPi) which contained PAP was identified in leaf extract. In this complex, the enzymatic activity of the toxin was strongly inhibited. This protein complex had a pI lower than that of PAP and was separated from free PAP by a preparative native gel electrophoresis. PAPi had an apparent molecular mass of 57 kDa and was dissociated by heating for 5 min at 80 degrees C or by treatment by alkaline or acidic pH or by 7 M urea. The other components involved in the complex remain unknown.