Summary: Two subtypes St(+) and St(-) of human lambda polypeptide chains were distinguished by immunochemical tests. Five of 27 Bence Jones proteins contained antigenic determinants that were not detected in the other 22 proteins. The distinctive antigenic sites were demonstrated with a heterologous rabbit antiserum produced to a lambda Bence Jones protein. The St(+) lambda chain subtype is present in light chains from pooled normal human immunoglobulin G. The results of comparative tryptic peptide mapping suggest that the specific St(+) determinants are localized on the variable (N-terminal) half of the lambda chains. St(+) lambda chains have been found to be Oz(-) (arginine residue at position 190 of the C-terminal half of the chain). Twenty-three per cent of the St(-) proteins were Oz(+) (lysine residue at position 190) and 77% of the St(-) proteins were Oz(-).