Cerebrospinal fluid from patients with aneurysmal subarachnoid hemorrhage induces the elevation of cytosolic free calcium [Ca2+]i in cultured vascular smooth muscle cells. We have purified a [Ca2+]i elevating factor from cerebrospinal fluid of a patient with subarachnoid hemorrhage due to aneurysm rupture. The calcium-elevating protein factor was purified to homogeneity by ammonium sulfate precipitation and a combination of Mono Q, Superose 12, and Mono S columns using liquid chromatography. Fifteen microgram of the purified protein was obtained from 340 mg of cerebrospinal fluid proteins and the molecular mass of the protein was estimated to be 81 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis. It was also shown that the purified protein was cross-reactive with anti-human transferrin antibody. These results suggested that transferrin may be involved with the cerebral vasospasm after subarachnoid hemorrhage.