Corneas and tendons dissected from 17-day-old chick embryos were labeled with [35s]methionine in the presence of 0.3 mMα.α'-dipyridyl. The unhydroxylated,35s-labeled proαchains andαchains were isolated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The proαandαchains were then subjected to peptide-map analysis by proteolytic digestion with trypsin andα-chymotrypsin, papain or proteinase K. The peptide-maps derived from cornea and tendon proα1(I) chains are identical. Similar results were obtained from cornea and tendonα1(I) chains. There were differences in the peptide maps derived from cornea and tendon proα2(I) chains. However, no difference was observed inα2(I) chains. These results suggest that cornea and tendon proα1(I) chains are probably identical in primary structures, whereas the cornea proα2(I) chain may be different from the tendon proα2(I) chain within pepsin sensitive regions of the procollagen molecule. The reason for difference in the peptide-maps of proα2(I) chains remain unknown. One of the possible explanations is the variation of posttranslational modification within the propeptides of the proα2(I) chain. However, this hypothesis needs to be further investigated. Nevertheless, the finding that the peptide-maps ofα2(I) chains from tendons and corneas are identical fail to support the two genes hypothesis for proα2(I) chains.