The complete amino acid sequence of antiviral protein from the seeds of pokeweed (Phytolacca americana) has been determined. This has been done by the sequence analysis of peptides derived by enzymatic digestion with trypsin, pepsin, and lysylendopeptidase, as well as by chemical cleavage with cyanogen bromide. The protein consists of 261 amino acid residues containing two disulfide bonds and has a calculated molecular mass of 29167 Da. The two disulfide bonds connect Cys-34 to Cys-258 and Cys-84 to Cys-105. Comparison of this sequence with the sequence of the ricin A-chain shows that there are identical residues at 76 positions in the two molecules (30% identity), having an extended region of highly conserved sequence at positions 170-183 (IQMXSEAARFXYIE). In contrast, the internal regions at positions 77-119 and 141-169 and the C-terminal 15 amino acid residues are less homologous in the two proteins.