Cis-hydroxyproline has been found to arrest the growth of rat mammary adenocarcinomas in vivo in the absence of general toxicity. The proline analog was shown to be highly selective in blocking the production of type IV collagen by the tumor cells in culture indicating that synthesis of this protein is essential for the growth and/or survival of such tumors. These results have been confirmed with two additional proline analogs, azetidine 5-carboxylate and 4-thioproline which are selective inhibitors of collagen synthesis and which block the growth of mammary adenocarcinoma cells in culture and in vivo. Azetidine 5-carboxylate and thioproline caused tumor regression after treatment of animals with 50 or 100 mg/kg SC, twice daily for 10 days. Histological examination revealed no pathological effects on cartilage, skin, intestines, liver, bone or spleen at these dosages. Nor was the growth of the animals affected. The tumor regression may have been mediated via effects of these proline analogs on collagen synthesis since the relative amount of collagen in the treated vs control tumors was reduced.