Six reference strains of Trypanosoma rangeli from different days of growth in axenic cultures were assayed for susceptibility to complement-mediated lysis by non-immune guinea-pig serum. Their authenticity was also confirmed by isoenzyme analyses. Parasites were incubated with 25% active or 68°C-inactivated serum (37°C, 30 min) for all tests; thereafter the lysis rates were determined. Highly variable lysis percentages were observed among T. rangeli strains and in the same stock at different growing days. In a few assays, three strains (Macias, R-1625 and Choachi) presented total or very high resistance. The others (H-14, San Agustín and SC-58) were generally most susceptible, and could reach lysis rates as high as Trypanosoma cruzi. After incubation with active sera, the epimastigotes were usually the predominant stages, being followed by spheromastigotes and/or transitional forms. Those stages and trypomastigotes could also be partially susceptible. In four strains, the short epimastigotes were more resistant to lysis than the long ones. Experiments with C3-deficient serum displayed total or partial participation of the alternative-complement pathway in T. rangeli lysis. This study confirmed the variable complement sensitivity of T. rangeli, which can be related to its intraspecific heterogeneity, to the remarkable complexity of its life-cycle stages, and to the methodology employed.