CM-cellulose and Agarose chromatography of denatured rat skin collagen show it to contain α1-chains in two distinct molecular weight classes: (α1) l , 1 × 10 5 daltons; (α1) h, 1.1 × 10 5 daltons. Pulse labeling in vivo with 3H-proline (G) for short periods, 90 and 120 minutes, shows the (α1) h-chains of acid soluble collage to have nearly double the specific activity of the (α1) l -chains, indicating a precursor-product relationship. However, unlabeled (α1) h has been isolated from the skins by successive extractions, indicating that conversion of (α1) h to (α1) l is neither immediate nor, possibly, complete. Since (α1) h is smaller than the pro-α1 chains of nascent procollagen, it is clear that procollagen is not cleaved in vivo in a single step to collagen. Heightened aggregation properties of the intermediate collagens containing (α1) h suggest that they may have a particular role in fibrillogenesis.