The effect of moderate and severe undernutrition on the immune response to EDIM rotavirus was studied in mice. A moderate state of undernutrition was produced by feeding weanling mice either 15 or 30% less diet than the control group had eaten on the previous day, for 7 weeks. After 6 weeks, mice were given 30 µl/mouse of EDIM rotavirus orally and the antibody production was measured 1 week later. The delayed-type hypersensitivity (DTH) response to picryl chloride (PC) was measured as an index of cell-mediated immunity. For severe undernutrition, weanling mice were fed either 30 or 50% less diet than the control group had eaten on the previous day, for 12 weeks. After 11 weeks of feeding, animals were given 30 µl/mouse of EDIM rotavirus either orally or intramuscularly and the antibody production was measured 1 week later. In moderate undernutrition, the serum antibody levels specific to rotavirus and the DTH response to PC were normal in all dietary groups compared with the control group. In severe undernutrition, the antibody levels following oral challenge of rotavirus were similar in all dietary groups. There was a significant impairment of serum antibody levels following intramuscular challenge in the group fed 50% less than the control group. It is concluded (1) that a balanced reduction in food intake does not impair the immune response, unless severe restriction is maintained for a long period of time, and (2) that the antibody response varies with the route of immunizatio