Abstract. Calreticulin is an ubiquitous and highly conserved high capacity Ca2+-binding protein that plays a major role in Ca 2÷ storage within the lumen of the ER. Here, using L fibroblast cell lines expressing different levels of calreticulin, we show that calreticulin plays a role in the control of cell adhesiveness via regulation of expression of vinculin, a cytoskeletal protein essential for cell-substratum and cell-cell attachments. Both vinculin protein and mRNA levels are increased in cells overexpressing calreticulin and are downregulated in cells expressing reduced level of calreticulin. Abundance of actin, talin, cx 5 and 131 integrins, pp125 focal adhesion kinase, and a-catenin is not affected by the differential calreticulin expression. Overexpression of calreticulin increases both cell-substratum and cell-cell adhesiveness of L fibroblasts that, most surprisingly, establish vinculin-rich cell-ceU junctions. Upregulation of calreticulin also affects adhesion-dependent phenomena such as cell motility (which decreases) and cell spreading (which increases). Downregulation of calreticulin brings about inverse effects.Cell adhesiveness is Ca > regulated. The level of calreticulin expression, however, has no effect on either the resting cytoplasmic Ca 2+ concentration or the magnitude of FGF-induced Ca 2÷ transients. Calreticulin, however, participates in Ca 2+ homeostasis as its level of expression affects cell viability at low concentrations of extracellular Ca 2÷. Consequently, we infer that it is not the Ca 2÷ storage function of calreticulin that affects cell adhesiveness. Neither endogenous calreticulin nor overexpressed green fluorescent proteincalreticulin construct can be detected outside of the ER. Since all of the adhesion-related effects of differential calreticulin expression can be explained by its regulation of vinculin expression, we conclude that it is the ER-resident calreticulin that affects cellular adhesiveness.