Log phase promastigotes from Sb-S strains S1 (closed squares, -■-), S2 (open squares, -□-) and Sb-R strains R1 (inverted closed triangles, -▼-), R2 (upright open triangles, -Δ-) and R3 (open circles, -○-) were exposed to increasing concentration of H2O2 (0 - 1000 μM) at 37°C for 1 h, probed with dichlorodihydrofluorescein diacetate (H2DCFDA) at 37°C for 45 for minutes and dichlorofluorescein (DCF) fluorescence was analysed as described in Materials and Methods. Data were expressed as MFC ± SEM of at least three independent experiments in duplicates.