cis-4-Hydroxy-L-proline, a proline analogue, is an inhibitor of collagen production. cis-4-Hydroxy-L-proline could inhibit fibroblast growth by preventing the deposition of triple-helical collagen on the cell layer. cis-4-Hydroxy-L-proline also depresses the growth of primary N-nitrosomethylurea-induced rat mammary tumors