The possibility of transdifferentiation of macrophages into fibroblasts which could be at the origin of fibrotic tissue in schistosome-infected mice was studied using immunocytochemical techniques. Macrophage cell samples extracted from the peritoneal cavity of schistosome-infected mice were fractionated on a Percoll gradient. The cultures were purified by treatment with a trypsin solution to eliminate any fibroblasts possibly collected along with the macrophages. Immunocytochemical methods were then used to characterize the cells at differentpoints in time. The fibroblastic property of the morphologically transformed cells was confirmed by their positive labeling with the anti-procollagen antibody. However, these cells still possessed the mac-1 and mac-2 antigens which characterize the monomacrophage line.