We have investigated the origins of the two major size classes of delta-crystallin polypeptides (48 kDa and 50 kDa on sodium dodecyl sulfate-urea-polyacrylamide gels) in the chicken lens. Both the 48-kDa and 50-kDa polypeptides were produced by cell-free translation in a salt-dependent (Na+, K+, acetate and Cl-) pattern from mRNA derived from a cloned delta 1-crystallin cDNA. The salt-dependent alteration in the ratio of cell-free synthesis of the 48-kDa to 50-kDa delta-crystallin polypeptides was greatly enhanced by capping of the delta 1 mRNA. Translation of the delta 1 mRNA containing a premature termination codon suggested that the larger delta-crystallin band contains two polypeptides which differ in their N-terminal one-third. In vitro transcription/translation analysis of several mutant delta 1 cDNA clones and immunoblot analysis of authentic delta-crystallin with antisera raised to peptides contained in delta-crystallin showed that neither alternative initiation at two in-phase AUG codons nor alternative termination at sites on the delta 1 mRNA are responsible for generating the two sizes of the delta-crystallin polypeptides. Taken together our data suggest (but do not prove) that delta-crystallin heterogeneity is generated by co-translational modification of the primary polypeptide encoded in the delta 1-crystallin gene.