AbstractSPARC (secreted protein, acidic and rich in cysteine), also known as osteonectin, is an extracellular Ca+2‐glycoprotein that inhibits the incorporation of [3H]‐and delays the onset of S‐phase in synchronized cultures of bovine aortic endothelial (BAE) cells. This effect appears not to be dependent on the functional properties of SPARC associated with changes in cell shape or inhibition of cell spreading. In this study we investigate the conditions under which cell cycle modulation occurs in different types of cells. Human umbilical vein endothelial cells, a transformed fetal BAE cell line, and bovine capillary endothelial cells exhibited a sensitivity to SPARC and a cationic peptide from a non‐Ca+2‐region of SPARC (peptide 2.1, 0.2—0.8 mM) similar to that observed in BAE cells. In contrast, human foreskin fibroblasts and fetal bovine ligament fibroblasts exhibited an increase in the incorporation of [3H]‐in the presence of 25 μM—0.2 mM peptide 2.1; inhibition was observed at concentrations in excess of 0.4 mM. This biphasic modulation could be further localized to a sequence of 10 amino acids comprising the N‐terminal half of peptide 2.1. A synthetic peptide from another cationic region of SPARC (peptide 2.3) increased [3H]‐incorporation by BAE cells and fibroblasts in a dose‐dependent manner. In endothelial cells, a stimulation of 50% was observed at a concentration of 0.01 mM; fibroblasts required ∼ 100‐fold more peptide 2.3 for levels of stimulation comparable to those obtained in endothelial cells. The observation that SPARC and unique SPARC peptides can differentially influence the growth of fibroblasts and endothelial cells in a concentration‐dependent manner suggests that SPARC might regulate proliferation of specific cells during wound repair and remode