A whole blood microculture assay is described for measuring lymphocyte reactivity to mitogenic and antigenic stimulants. This assay employs heparinized whole blood, serum-free culture medium, microtiter plates, and a Multiple Automated Sample Harvester (MASH). When this assay is compared to other leukocyte assays, its major advantages are as follows: the utilization of fewer lymphocytes per microculture, thus reducing the amount of blood required per test while increasing the number of test agents and replicate cultures which can be employed in any given experiment; the conservation of mitogens, antigens, drugs, enzymes, hormones, lymphokines and other test agents, some of which are expensive or difficult to prepare in large quantities; the elimination of lymphocyte isolation and purification procedures which may disrupt the relative proportion of T [thymus-derived] cells, B [bone marrow-derived] cells and antigen-processing cells; and the application of an automated harvester which simplifies and expedites procedures required for processing cells for liquid scintillation counting.