After a brief examination of the recent literature on plasma cell leukemia, the Authors describe the clinical features and the ultrastructural findings of the peripheral blood plasma cells, examined with transmission (T.E.M.) and scanning (S.E.M.) electron microscopes, of two patients with acute plasma cell leukemia. Both of them had a previous history of myeloma. T.E.M. confirmed the diagnostic value of the asynchronous plasma cells, and S.E.M. showed the characteristic microvilli and blebs, previously observed.