AbstractSerum alpha‐1 acid glycoprotein levels were measured in 27 patients with idiopathic peripheral retinal vasculitis (Eales' disease) and 31 healthy subjects by single radial immunodiffusion technique. The levels were found to be significantly increased in both moderate and severe types of the disease. The serum levels of this protein paralleled the severity of the disease. The increased alpha‐1 acid glycoprotein in serum showed a significant fall to near normal levels during the quiescent stage. Our observations support the hypothesis that idiopathic peripheral retinal vasculitis is an immune mediated disease. It is proposed that serum alpha‐1 acid glycoprotein may be considered as a reliable parameter of the activity and the degree of severity of the disease, as well as an useful tool for monitoring the efficacy of trea