Lymphocyte fractions of three healthy donors were tested for cytotoxicity in a 48‐h assay on several tumor‐derived lines. Analysis of surface markers indicated that a non‐T non‐B fraction comprising about 50% Fc‐receptor‐positive cells was most potent. The cell yield in this fraction was on the average 6.5% of the non‐fractionated population. Elimination of SIg positive cells did not influence the cytotoxic potential. Pure T cells isolated with E‐rosetting subsequent to passage on nylon wool column had no or very low cytotoxicity. We had indications that cells without conventional markers —“null” cells—were also cytotoxic.