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Previous studies established that some isolates of Giardia differ antigenically. In order to determine if antigenic differences resulted in altered biological behavior of host immune responses, two antigenically distinct isolates, WB and GS-E, were used to infect gerbils, and the course of infection, resistance to reinfection, and host humoral responses were measured. Maximum numbers of trophozoites were recovered on day 14 from the intestine of gerbils infected with both isolates, but by day 28, 75% of WB-infected gerbils were free of infection, while GS-E-infected animals continued to be infected until day 42. After curative metronidazole therapy, animals were challenged with the homologous or heterologous isolates. Gerbils previously infected with WB were resistant to challenge with WB and GS-E, while previously GS-E-infected gerbils were more resistant to challenge with the homologous isolate. Antibody responses were measured by ELISA to both surface and cytosol antigens and by IFA to the surface of Giardia. By IFA there was a greater reactivity using the homologous isolate, but with ELISA this was not as apparent. Complement independent cytotoxicity of sera was additionally tested against both isolates. Sera from WB-infected gerbils were cytotoxic to both WB and GS-E whereas sera from GS-E-infected gerbils were cytotoxic to GS-E only. These studies demonstrate that Giardia possessing different surface antigens have different patterns of infection and induce qualitatively and quantitatively different immune responses. Cytotoxicity of sera, most likely antibodies, correlated best with the development of resistance.