We conducted an ultrastructural study in 22 cases of B-lymphoproliferative disorders in leukaemic phase bearing the t(11;14) translocation. The features of peripheral blood leukaemic cells in nine cases of mantle cell lymphoma (MCL) were compared to those diagnosed as B-prolymphocytic leukaemia (B-PLL) (five cases), splenic lymphoma with villous lymphocytes (SLVL) (four cases), lymphoplasmocytic lymphoma (LPL) (one case), chronic lymphocytic leukaemia with > 10% prolymphocytes (CLL/ PL) (one case) and unclassified B-non Hodgkin's lymphoma (B-NHL) (two cases). The ultrastructural characteristics were also compared to those present in B-NHL without t(11;14), including cases of follicular centre lymphoma (FCL). This study shows that MCL has distinct ultrastructural features including a cleaved or indented nucleus with an even heterochromatin distribution, an absent or inconspicuous nucleolus, low N/C ratio, abundant mitochondria, a well developed Golgi zone, profiles of endoplasmic reticulum and centrioles. This pattern clearly differs from that found in FCL cells. The nuclear pattern of MCL cells also differed from the cells in the other disorders with t(11;14), but shared an organelle-rich cytoplasm, and features which were not apparent in cases without t(11;14). The cytoplasmic changes observed in cells bearing t(11;14) suggest increased cellular activity which may relate to the chromosome translocation and the resulting over-expression of bcl-1.