Ancylostoma caninum--mouse model was employed to study the cellular cooperation in the adoptive immune response. The syngeneic recipient mice were intraperitoneally injected once or twice with mixtures of thymus and bone marrow cells from infected (with 500 or 2000 larvae) and uninfected donors. The experimental recipients expelled and/or destroyed the challenge larval burden more readily and at a greater rate than the controls with unsensitized cells. The cooperation between sensitized thymus and bone marrow cells was, thus, found to be exposed in a better manifestation of adoptive immune response than either of these two alone. The cellular elements of delayed hypersensitivity after combining with the antibodies of humoral system could elicit a much better response in these recipients.