SummaryHumoral and cellular immune responses have been evaluated in two inbred strains of mice which differ markedly in their susceptibility to infection withGiardia muris.Serum IgG and IgA antibody levels and IgA levels in intestinal washes were determined by a solid‐phase radioimmunoassay usingG. murisantigen prepared by sonication of trophozoites, while cell‐mediated immunity was assessed by a radiometric ear‐assay for delayed‐type hypersensitivity. Following infection of BALB/c mice (resistant) and C3H/He mice (susceptible), the IgG and IgA antibody levels in serum progressively increased over the period of study with C3H/He mice having significantly higher titres of IgA antibodies than BALB/c late in the infection. Systemic immunization withG. muristrophozoites resulted in high titres of IgG antibodies in the serum. IgA antibodies were detected in intestinal washes 2 weeks after infection with a subsequent fall in levels in BALB/c mice but a progressive increase in levels in C3H/He mice. Prior immunization resulted in IgA antibodies being detected earlier in the intestinal washings after a challenge infection. Delayed‐type hypersensitivity toG. murisantigens could not be detected during an infection but a positive response was elicited following antigen priming in mice pretreated with cyclophosphamide. The immune responses evaluated in this study were assessed using a wholeG. muristrophozoite sonicate and variations in the quantitative aspects of the responses did not account for observed differences in the course of infection in the two strain