Osteonectin/SPARC regulates cellular secretion rates of fibronectin and laminin extracellular matrix proteins. Osteonectin (OTN) has been implicated in controlling cell adhesivity onto substratum and extracellular matrix (ECM) remodeling. Significant amounts of OTN were synthesized not only by normal fibroblasts and endothelial cells, but also by HT-1080 fibrosarcoma and MG-63 osteosarcoma cells. Levels of secreted OTN were likely to be slightly elevated by the addition of exogenous placental laminin (LN), but not by supplementation of plasma fibronectin (FN). Exogenously supplemented purified bone OTN was not apparently incorporated into the ECM of the adhering cells and had no effect on cell spreading and growth, whereas secretion of type I collagen or FN in the tumor cells was moderately diminished in the presence of soluble OTN. Concentration-dependent down-regulation of cellular LN secretion appeared to be most significant, suggesting that OTN participates in regulating extracellular secretion of ECM components in the cells either with or without the ability to synthesize cellular OTN.