The reproducibility of the measurement of concanavalin A (Con A)-inducible suppressor cell activity in human peripheral blood lymphocytes was examined. Suppressor cells were activated for 24, 48, hr 72 hr with 3, 6, and 12 µg of Con A (Con A in concentrations greater than 12 µg/ml was toxic to the lymphocytes) per ml and suppression of the proliferative response of autologous lymphocytes to varying concentrations of Con A (3, 6, and 12 µg/ml) was measured. No single set of conditions consistently produced optimal suppression. Assays performed concurrently using lymphocytes from the same subject produced comparable suppressor cell activity; however, considerable variability in suppressor cell activity was observed under all conditions tested when five normal subjects were examined on as many as four separate occasions. The variability was reduced but not eliminated by reporting the data for each assay in terms of a peak suppression value determined from multiple sets of conditions. The results suggest that small differences in suppressor cell activity between patient groups may be masked by the intrinsic variability of the assay system.
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