After placement in mice of PZQ-sensitive and -insensitive S. mansoni isolates obtained from villagers responding and not responding to PZQ, parasitological criteria reflecting their biological development and also the host anti-immature worm immunoglobulin isotypes were examined 8 and 10 weeks post infection. Hepatic granuloma diameter, hepatic histopathological changes and immunolocalization of IgG and IgM on the surface of PZQ-sensitive and -resistant worms were also examined 10 weeks post infection. Data showed that parasitological criteria were not significantly different between mice infected with the PZQ-sensitive and -insensitive S. mansoni isolates. As regards serum immunoglobulins, in mice infected with the PZQ-insensitive S. mansoni isolate, IgG and IgG1 were significantly (p<0.05) lower 8 and 10 weeks post infection, respectively (1.41+/-0.07 and 1.08+/-0.10 and 1.35+/-0.06 and 1.09+/-0.07) than in mice infected with the PZQ-sensitive S. mansoni isolate (1.73+/-0.15 and 1.38+/-0.10 and 1.73+/-0.17 and 1.54+0.21) after the same observation periods. IgM level was nearly the same while IgE was lower than that recorded in mice infected with the PZQ-sensitive S. mansoni isolate. IgG immunofluorescence was also lower (60%+/-6.78) on the surface of resistant worms than that of sensitive worms (66.6%+/-5.27); meanwhile, hepatic granuloma diameter was significantly larger (296.5+/-3.0 vs 283.6+/-4.0) in mice infected with the PZQ-insensitive S. mansoni isolate with higher percentage of intact eggs. Differences in the immunogenic make up of PZQ-sensitive and -insensitive S. mansoni isolates qualitatively and/or quantitatively favoring a certain Th cell subpopulation response could be the underlying reason for such differences recorded in the host immunoglobulin isotype response and also the egg-induced hepatic histopathological changes.