The primary structure of a precursor protein that contains the toxic (A) and galactose-binding (B) chains of the castor bean lectin, ricin, has been deduced from the nucleotide sequence of cloned DNA complementary to preproricin mRNA. A cDNA library was constructed using maturing castor bean endosperm poly(A)-rich RNA enriched for lectin precursor mRNA by size fractionation. Clones containing lectin mRNA sequences were isolated by hybridization using as a probe a mixture of synthetic oligonucleotides representing all possible sequences for a peptide of the ricin B chain. The entire coding sequence of preproricin was deduced from two overlapping cDNA clones having inserts of 1614 and 1049 base pairs. The coding region (1695 base pairs) consists of a 24-amino-acid N-terminal signal sequence (molecular mass 2836 Da) preceding the A chain 267 amino acids, molecular mass 29 399 Da), which is joined to the B chain (262 amino acids, molecular mass 28 517) by a 12-amino-acid linking region (molecular mass 1385 Da).