A miniaturized method for the mixed leukocyte culture (MLC) test is described which results in substantial savings in cells, medium, and time. The test is done in microtiter plates using 0.2-ml culture volumes. Concentrations of responding and stimulating cells are varied, depending on the experiment. Significant discrimination between isogeneic and allogeneic mixtures is possible when cultures are labeled for a 16–20-hr period starting at 48 hr. The method measures MLC identity, as previously defined, and quantitates in a manner significantly correlated with the macro method.