SummaryThe phagocytosis ofTrypanosoma cruziblood forms by mouse peritoneal macrophages is significantly enhanced by sera from chronic chagasic patients, rabbits and mice presenting ‘lytic antibodies’ (LA) which are associated with resistance and active infections as well as ‘conventional serology antibodies’ (CSA) which are immunoglobulins involved in the positivity of serological diagnostic tests. The phagocytosis rate, however, is not influenced by sera from mice immunized withT. cruziantigen or chagasic patients submitted to specific treatment, both displaying only CSA but not LA. The efficacy of LA in increasing phagocytosis is related to their ability to bind to epitopes of living trypomastigotes, a property lacking in CSA that bind only to fixed parasites. This phenomenon is apparently the reason for the low effectiveness of antigens used for vaccination in Chagas' disease which only induce CSA, immunoglobulins apparently unable to mediate a number of regular effector immune mechanisms such as complement‐mediated lysis, antibody‐dependent cell cytotoxicity and p