AbstractProline, a critical substrate for collagen synthesis, is increased in liver undergoing fibrosis. In mice with schistosomiasis, the incorporation of proline into collagen occurs within liver granulomas. To study the interaction of liver cells and granulomas in the development of fibrosis, we assayed the enzymes that catalyze the formation and degradation of proline in isolated granulomas and liver.Two sequential enzymes involved in the formation of proline from arginine are ornithine‐δ‐aminotransferase and Δ1‐pyrroline‐5‐carboxylate (P5C) reductase. Activities of both these enzymes in granulomas were approximately 10% of those in liver, expressed on the basis of DNA content.The enzymes involved in degradation of proline are proline oxidase and P5C dehydrogenase; both are present in liver cells. In isolated granulomas, activity of proline oxidase was minimal, and P5C dehydrogenase activity was absent. These findings suggest that the metabolism of proline within granulomas differs greatly from that in liver cells. Earlier studies showed that exogenous proline enters granulomas and is rapidly incorporated into collagen. The combined findings raise the possibility that granulomas serve as a proline trap. Proline can enter this compartment and can be incorporated into collagen; however, proline within granulomas cannot readily be diverted into other pathways by