Mouse erythrocyte (M) rosette formation and colchicine sensitivity were compared for their ability to differentiate chronic lymphocytic leukaemia (CLL) from B cell non-Hodgkin's lymphoma (NHL) with overspill. Twenty-two cases of CLL and eight of NHL were studied along with 31 normal adults. Results from the patients in both tests differed significantly from the controls but colchicine sensitivity failed to differentiate them further. M Rosettes, on the other hand, while increased in some patients with NHL were, without overlap, much more numerous in those with CLL, and clearly distinguished the two conditions. A significant autolymphocytotoxic effect of plasma from both study groups was also noted which was not found in the controls.