dium (DMEM) supplemented with 2% fetal calf serum (16 hr, 34 C, 10% CO2 in air), at a density of 2 X 106 parasites/ml. Ninety percent of the forms became motile trypomastigotes. Inoculation at A/Sn mice with 1 X 106 purified amastigotes (i.p.) killed all animals within 15 days PI. The host tissues used as sources of parasites in this study were liver and spleen. These were easily disrupted without the need for enzymatic treatment. Thus, potential alterations of the parasites' cell surface were reduced. However, isolation of amastigotes from muscle tissue (myotropic strains) requires the use of enzymes and centrifugation of the cells in sucrose density gradients. The latter method decreases the yield by ium (DMEM) supplemented wi h 2% fetal calf erum (16 hr, 34 C, 10% CO2 in air), at a density f 2 X 106 parasites/ml. Ninety percent of th about 30% (Gutteridge et al., 1978, Parasitology 76: 159-176). The major advantage of our procedure for isolating T. cruzi amastigotes is the very high yield of viable parasites without enzymatic treatment of the host tissue. This technique can provide a valuable tool whenever a large number of amastigotes are needed for antigen preparation or biochemical studies. This investigation was supported by CNPq grants 2222.8.079/80 and 2222.8.141/80. Dr. Katzin was supported by an AMK fellowship of CONICET (Argentina). Dr. Abrahamsohn acknowledges the initial gift of Percoll from Pharmacia-Uppsala.