cyr61 and fisp12 are homologous immediate-early genes that are transcriptionally activated upon growth factor stimulation in fibroblasts. Their gene products belong to an emerging family of secreted proteins with a high degree of sequence homology, including conservation of all 38 cysteine residues in their secreted portions. We have recently shown that Cyr61 is an extracellular matrix (ECM) signaling molecule that promotes cell proliferation, migration, and adhesion. We describe herein the first purification of the Fisp12 protein and we compare the activities of purified Cyr61 and Fisp12, their metabolism, targeting, and their localization during development. Although Fisp12 is the mouse homolog of the human connective tissue growth factor (CTGF), it has no detectable mitogenic activity by itself. Rather, Fisp12 enhances fibroblast growth factor-induced DNA synthesis. The activities of Fisp12 and Cyr61 are nearly indistinguishable in three cell types tested: fibroblasts, endothelial, and epithelial cells. Both proteins are found in the ECM, although Cyr61 associates with the ECM more strongly and binds heparin with higher affinity. Fisp12, but not Cyr61, is also found in the culture medium, suggesting that Fisp12 might be able to act at a distance from its site of secretion, whereas Cyr61 might act more locally. Both secreted proteins are internalized and degraded through the lysosomal pathway, suggesting interaction with cell surface receptors. Both Cyr61 and Fisp12 are found in the placenta and the circulatory system as detected by immunohistochemistry, whereas Cyr61, but not Fisp12, is found in the skeletal and nervous systems. Fisp12, but not Cyr61, is found in secretory organs. Taken together, we propose that Cyr61 and Fisp12 are both signaling cell adhesion molecules that have similar or overlapping activities, and their differential sites of localization and targeting may dictate specificity in their biological roles.