A semimicromethod for performing mixed leukocyte culture (MLC) tests was devised, permitting a 10–20-fold increase in the number of tests which can be made from any given sample. Optimization of various parameters in the test and identification of the key factors which would allow this reduction in the number of test cells without loss in resolution were investigated. The use of lymphocytes which were almost free of red cells and of granulocytes, micropolyethylene tubes, and high levels of3H-thymidine were the principal modifications permitting the reduction in the number of cells to 0.05 X 106. Lymphocytes obtained from transport bags mailed to the laboratory were also effective. Ten pairs ofHL-A-identical siblings gave an average stimulation ratio of 1.12, whereas the same siblings reacted to one-haplotype different related donors with a mean ratio of 9.64 and to unrelated persons with a mean ratio of 25.72.