We developed a method to asses the infectivity of Trypanosoma cruzi trypomastigotes for cultured Vero cells, after incubation under different conditions with human leucocyte populations. Total leucocytes, polymorphonuclear (PMN) and mononuclear (MN) cells, from either infected (INF) or cardiomyopathic (CDM) Chagas patients were tested in the presence or absence of autologous serum. Both patient groups had positive complement fixation tests (CFT) for T. cruzi and these were compared with a group of normal controls with negative CFT. Serum alone was found to cause reductions in the infectivity of the trypomastigotes, particularly in the case of CDM. However, in the control and INF groups, a significantly greater effect was observed with combinations of leucocytes and serum. The inhibition caused by cells plus serum was significantly greater in INF and CDM patients than in normal controls. In addition, we evaluated the penetration of trypomastigotes into the different types of leucocytes and found a significantly greater penetration of INF leucocytes at 15 min, compared to the controls, in the presence of both autologous and homologous normal serum. However, after 60 min of incubation the differences were not statistically significant.