Assays of the exophthalmos-producing substance (EPS) in the serum of patients with progressive exophthalmos were performed using Fundulus hetroclitus, an Atlantic minnow. The assay consists of measurement of the exophthalmic response in fish after injection of the patient''s serum. A variety of factors affecting the responsiveness of the fish were partly controlled by the use of standard pituitary extract. The serum of certain patients produced pronounced exophthalmos in the fish. There was no strict correlation between the degree of response in the fish and the severity of clinical progressive exophthalmos. A better correlation was observed in patients early in the development of the disease. Several impressive responses were elicited by the serum of patients with Graves'' disease preceding the onset of severe exophthalmos. Late in the course of progressive exophthalmos no high assay values were found. Mild degrees of response were occasionally elicited by the serum of normal subjects. Pituitary stalk section and cauterization of the anterior pituitary in a patient with extreme exophthalmos was followed by dramatic improvement in the eye symtoms and a fall in the EPS activity of the serum.