Using an electronmicroscopic peroxidase-antiperoxidase technique in combination with a three-step fixation procedure according to Willingham, the ultrastructural site of terminal deoxynucleotidyl transferase (TdT) could be shown in cells of a human lymphoblastic leukemia cell line (MOLT 4). TdT appeared in the cytoplasm of the cells associated with large membrane bound granules from 150 to 600 nm in diameter usually arranged in aggregates. TdT-positive spaces were completely segregated form the rest of the cytoplasm and seemed to be interconnected by a tubular system. No TdT was detected in the nuclei.