Pre-treatment by neuraminidase of lymphocytes obtained from peripheral blood of normal donors significantly enhanced E- and EAC-rosette formation. Of other lymphoid cells only spleen cells showed significant enhancement of E-rosettes. The EAC-rosettes slightly increased when the peripheral blood lymphocytes from patients with acute lymphoblastic leukemia or chronic lymphocytic leukemia and MOLT-4 lymphoid cells were pre-treated with this enzyme. The EAC-rosettes were not increased by neuraminidase treatment of phytohemagglutinin-induced blasts, thymus cells or spleen cells. Pre-treatment of peripheral blood lymphocytes with neuraminidase also increased the proportion of stable E-rosettes resistant to incubation at 37 degrees C and to vigorous shaking. Various concentrations of neuraminidase (1-100 U/ml) produced enhancement of E- and EAC-rosettes with the highest activity at 25 and 50 U/ml. Neuraminidase treatment of sheep red blood cells failed to increase the proportion of E-rosettes of peripheral blood lymphocytes. The increased rosette forming capacity induced by neuraminidase is probably related to changes in lymphocyte surface properties.