Pokeweed antiviral toxin is a plant protein which has been shown to have both antiviral and cytotoxic activities. The site of action for the toxin has been shown in cell-free lysates to be at the ribosomes and specifically the Ef-2 mediated translocation step to the elongation cycle during protein synthesis. The protein isolated from the leaves exists as two forms, pokeweed antiviral protein (PAP) at a molecular weight of 29 kDa and PAP II at a molecular weight of 30 kDa. Classical plant toxins such as ricin or abrin are composed of two functionally active fragments. The A chain contains the toxic activity and the B chain gives the toxin a cell recognition and binding function to facilitate transport across the cell membrane. The A chain is not active until it is internalized by the cell, where it halts protein synthesis. Pokeweed antiviral toxin is a hemitoxin which has the A chain activity but lacks a B chain. Without the B chain to mediate cell membrane binding, the toxin cannot enter the cell. By conjugating the protein to a monoclonal antibody or a lectin, the substance can be bound to a cell membrane and internalized where it acts to inhibit protein synthesis.