Experiments were done in vivo in order to compare the ability of Trypanosoma cruzi bloodstream trypomastigotes (BTr), amastigotes (Am), metacyclic trypomastigotes (MTr) and epimastigotes (Ep) to proliferate in the peritoneal macrophages of the following groups of A/Sn mice: actively immunized with increasing doses of living BTr (AIM); mice passively immunized with homologous immune serum (ISTM); mice passively transfused with normal homologous serum (NSTM) and normal mice (NM). Parasite load was evaluated by counting the number of infected peritoneal macrophages harvested on the 4th day after i.p. challenge with one of the forms. All four parasite stages infected NM. AIM seldom showed parasitized macrophages after challenge with any of the forms. ISTM always harboured numerous parasitized macrophages. The only form susceptible to humoral immunity was the Am.