The ultrastructure of T and B lymphocytes has been examined in long-term syngeneic chimaeras and age-control mice. Spleen cell suspensions from these mice were passed through glass wool columns to obtain pure lymphocyte populations. These cells were then separated into T and B lymphocytes by nylon wool columns, and their purity was tested by cytotoxicity assays with anti-phi serum. Electron microscopic observations on such separated T and B lymphocytes did not reveal morphological differences except when the cells were fully differentiated, either as mature (T2) cells or plasmacells. In particular, T2 cells showed a very high cytoplasmic density, attributable to the presence of a larger number of microfilaments with respect to immature (T1) cells. In long-term chimaeras a significantly larger number of T2 cells was found as compared to age-control mice, and this morphological observation is correlated with the differences in immune reactivity and leukemia incidence previously described in these mice.