A band of plasma protein which was seemingly specific in SHR was detected by polyacrylamide gel electrophoresis. This protein was identified as one of the normal serum proteins in the rat, and was shown to be increased even in young stroke-prone SHR (SHRSP), especially in those with stroke-lesions as determined by single radial immunodiffusion. No alteration of this band was observed in rats with renal infarction hypertension 3 weeks after operation. This band corresponded to α_1-globulin fraction, and the characteristic increment of this protein fraction may be utilized as a marker of SHRSP, although its pathophysiological role is not yet clarified.