An in vitro reaction between mouse lymphoid cells and target fibroblastic cells in wells of microtest plates is described, which appears to simulate the in vivo rejection of haemopoietic allografts. Lymphoid cells from non-immunized mice were incubated with L929 or skeletal muscle fibroblasts, the latter from mouse embryos, in 0.02 ml volumes for 18–24 hours. Cell detachment was the measure of cytotoxicity, as the remaining adherent fibroblastic cells were enumerated in plates stained with Giemsa's stain. Cytotoxicity was related to the ratio of lymphoid cells to target cells plated (250:1 was the minimal ratio resulting in consistent cell damage) and to the length of time the two cell populations were incubated together. Neither extrinsic complement nor phytohaemagglutinin was necessary for the response. The lymphoid cells remained cytotoxic following exposure to 1000 R of X-radiation and were only slightly inhibited by rabbit antimouse thymocyte serum. Lymphoid cells from suckling mice were less effective than those from adults. Cyclophosphamide treatment suppressed the function and/or numbers of effective lymphoid cells. Differentiation of competent lymphoid cells from transplanted marrow stem cells was not inhibited by previous thymectomy of the recipient mice. Lymphoid cells were not cytotoxic for syngeneic target cells and were cytotoxic for allogeneic target cells. (C3H/He × C57BL/10Cz) F1 lymphoid cells were effective against C57BL/10Cz, but not C3H/He, target fibroblastic cells, demonstrating immunogenetic specific `hybrid resistance'.