Sera from 9 patients with a primary thymic tumour but without myasthenia gravis (MG) at the time of thymectomy were examined for skeletal muscle antibodies. Antibodies to a citric acid extract (CA), associated with the presence of a thymic lymphoepithelioma in MG patients, were detected in 3 sera. AChR antibodies were detected in 5 sera, the concentration markedly elevated in 1 of them. Sarcolemmal and cross-striational antibodies were detected in 2 and 3 sera, respectively. Four of the 5 patients with thymic tumours other than a lymphoepithelioma had AChR antibodies, and 3 of them also had CA antibodies.