SUMMARYA large population of primitive basophilic mononuclear cells (immunocytes), capable of synthesizing DNA, occurs in the peripheral blood of patients with GBS. In this study, the electron-microscopic features of these circulating atypical mononuclear cells were correlated with their autoradiographic and light-microscopic characteristics. Circulating immunocytes in GBS have ultrastructural features which are consistent with their high metabolic activity and are indistinguishable from some of the cells infiltrating the nerve roots in GBS. This provides further evidence that these cells may play an important role in GBS pathogenesis.