Summary: The microchamber and the plaque-in-agar localized hemolysis techniques were used simultaneously to assay rabbit immune spleen cell suspensions for high hemolytic efficiency plaque-forming cell activity. Direct plaque-forming cell activities both for erythrocyte and for para-azo-benzenearsonic acid determinants were closely comparable by both methods, but plaque size frequency distribution data revealed the microchamber procedure to be potentially the more sensitive. Plaque size distribution data also indicated that for those cells recovered in suspensions the assay conditions permitted detection of the complete population of high hemolytic efficiency plaque-forming cells by either the microchamber or the plaque-in-agar techniques. Accordingly, for relevant plaque-forming cells recovered in suspension, relative assay efficiency closely approached absolute assay efficiency.