Parental strain rats were immunized in the footpads by F1 hybrid spleen cells. The draining lymph nodes were removed during the first 10 days after immunization and tested for mitogen responsiveness. Cell fractionation was performed by removal of macrophages, surface Ig positive, and F(c)-, and C3-receptor carrying lymphocytes. The depleted suspensions were tested in MLC to test the effect of lymphocyte subpopulations on the accelerated MLC response profile exhibited by the immune lymph node cells. Overall increase in mitogen stimulation was observed, but stimulation indices were slightly reduced as compared to non-immunized cells. Removal of phagocytic cells gave some increase in MLC response which was most pronounced when responder cells were tested during the first days after immunization. Depletion for non-T lymphocytes resulted in lowering of non-stimulated responder cell 14C uptake, whereas the specific accelerated MLC profile was unaltered or tended to improve acceleration. It is concluded from indirect evidence that T lymphocytes are the cell type responsible for the altered MLC kinetics, but that macrophages and non-T cells modify the response.