Immune sera obtained from either BALB/c mice (susceptible) at 7 weeks, or C57BL/6 mice (resistant), at 7 weeks after infection with L. m. amazonensis, were effective in increasing internalization of homologous promastigotes into starch-induced peritoneal macrophages (from both mouse strains). Both the internalization enhancing effect and the levels of anti-leishmanial antibody (ELISA) were removed from sera by absorption with heat-killed promastigotes. Sera at 1/200 dilution obtained from either mouse strain at 2 weeks after infection did not enhance parasite internalization into macrophages. The factors leading to susceptibility or resistance during leishmaniasis do not appear to be related to differences in antibody-mediated opsonic activity.