The 90 kDa heat shock protein (hsp9O) is a major phosphoprotein which associates various other cellular polypeptides such as actin, calmodulin, steroid hormone receptors and certain protein-kinases. Little is known about the function of hsp9O in recovery from stress. In this report, we describe a dramatic increase in the rate of both phosphate uptake and dephosphorylation of hsp9O in HeLa cells submitted to acute stresses. This increased turnover of hsp9O phosphate groups might reflect a greater protein binding activity of hsp9O in stressed cells.