Four cases of lymphocytic leukaemia were distinguished from CLL by surface marker analysis. CLL cells have relatively little SIg and have the mouse erythrocyte receptor. Cells in these cases had denser Slg and lacked the mouse erythrocyte receptor. The group presented a fairly uniform picture with onset in sixth or seventh decade, very high lymphocyte counts, splenomegaly (gross in three), heavy infiltration of marrow, relative resistance to chemotherapy and response to splenectomy. The lymphocytes were somewhat larger than those usually found in CLL and histological examination showed a quasi-nodular pattern in lymph nodes with diffuse infiltration of the spleen. Difficulties of classification are discussed. The addition of surface marker analysis to clinical and morphological parameters allows these cases of B-lymphocytic leukaemia (mouse erythrocyte-negative B leukaemia) to be distinguished clearly from chronic lymphocytic leukaemia. The mouse erythrocyte receptor is particularly useful in making this distinction.