Striational autoantibodies (StrAb) are a useful serologic marker of thymoma in patients with myasthenia gravis (MG). We compared a standard immunofluorescence method with a new enzyme immunoassay (EIA) for detection of StrAb. Retrospective testing of 264 stored sera by the two methods yielded well-correlated results (58 sera were positive by both assays; r = 0.8). For 104 patients with spontaneously acquired MG or thymoma, results were 100% concordant, of which 53% were positive. For 34 recipients of D-penicillamine, StrAb were found in 15% by EIA and in 6% by immunofluorescence. StrAb were detected in two of four bone marrow recipients by EIA and in one by immunofluorescence. Prospective testing of 434 fresh sera (of which 49 were positive by the two methods) yielded discordant results in only 4. Serial EIA quantitation of StrAb in two patients with MG and thymoma proved useful in monitoring immunosuppressant therapy and in a third patient predicted recurrence of the tumor. A high prevalence of StrAb was detected by both assays in elderly patients with spontaneous MG, but StrAb were more readily quantifiable by EIA. The EIA method proved to be highly sensitive and specific for detecting StrAb in patients with thymoma with and without MG, in patients treated with D-penicillamine, and in those with graft-versus-host disease after bone marrow transplantation.