An attempt was made to locate the biologically active component of the DLE in lymphocytes. The test was based on the recovery of sheep cell-rosetting capacity in trypsinized human lymphocytes (recovery assay). Comparisons of the extract from trypsinized leukocytes and the leukocyte supernatant (after trypsination ) yielded the following results: The peptide fraction detached from the cell surface by trypsin (30 min with 0.5 g trypsin/l at 37 degrees C) accounts for most of the TF activity of the whole lymphocytes. Of the two TF activities (fractions II and III), fraction III obviously stems from the cell interior because it cannot be liberated by trypsin. Fraction III is characterized by an unusually high UV absorption quotient (A 260/280), probably due to a large nucleotide content. Trypsination leads to the biologically active TF fraction going into the supernatant. Fraction II consists almost entirely of cell surface peptides. It is relatively easy to separate it cleanly, and it has a high level of biological activity (1 microgram/ml is still detectable).