Specific IgG anti-Babesia antibodies could be detected by an ELISA test in the serum of adoptively immunized mice which subsequently were infected with Babesia microti. Initially, higher IgG antibody levels were present in the recipients of either untreated immune spleen cells or mitomycin C-treated immune spleen cells compared with the values in control mice which received the infection alone. Subsequently, a marked anamnestic response occurred only in mice which received untreated immune spleen cells and not in the recipients of mitomycin C-treated cells, demonstrating a requirement for proliferation of B memory cells in the anamnestic response. When mice were injected with either T or B enriched spleen cell subpopulations and infected with B. microti, the anamnestic antibody response in each animal correlated well with protection against infection. The stimulation of the adoptively transferred immune spleen cells in recipient mice by injection of B. microti antigen fractions prepared by DEAE cellulose chromatography increased the level of antibody production and protection against infection in these mice. A highly significant correlation between these two parameters could be demonstrated for certain antigen fractions.