Calreticulin is a highly conserved protein with a relative molecular weight of 46,000, and is mainly located in the endoplasmic reticulum. Calreticulin was first characterized as a calcium-binding protein in the endoplasmic reticulum, but since then other functions of calreticulin have been characterized, including chaperone and lectin properties, and regulation of integrin and nuclear hormone receptor activity. We have investigated the interactions between purified human placental calreticulin and native and denatured proteins. Our results show that calreticulin binds to denatured proteins in a time- and pH-dependent manner, which at physiological pH is dependent on divalent cations. The binding was dependent on the state of the denatured protein, and was highly sensitive to the ionic composition of the environment, being strongly inhibited by phosphate-containing compounds. Calreticulin did not seem to distinguish between denatured glycosylated and non-glycosylated proteins, and was found to bind to native basic proteins, presumably by sheer electrostatic forces.