In order to utilize the mixed lymphocyte reaction (MLR) as an assay of T-lymphocyte competence, pools of target lymphocytes obtained from different individuals are used to increase the magnitude and decrease the variation of the in vitro response. We evaluated variations in MLR response due to variations in target cell populations. Response increased with an increased target/responder cell ratio. Peak response occurred with a target/responder cell ratio of between 1:1 and 1:4. Response to a pool of lymphocytes from different individuals increased as the number of individuals contributing to the pool increased. Peak stimulation occurred with three to four different donors to the target cell pool. Stimulation produced by pooled target cells resulted in a higher mean index of stimulation and decreased variation of response as compared to stimulation produced by target cells from individual donors. Stimulation produced by pooled target cells was approximately equal to the sum of the stimulation produced by each of the target cell populations acting alone. These findings indicate a practical method of modifying the MLR as a test of T-lymphocyte function.