Lymphocytes forming spontaneous rosettes with sheep red blood cells (RFL) have been studied in normal individuals and patients with malignant lymphomas. Rosette formation was temperature dependent and was inhibited by sodium azide or trypsin treatment of -, but not by preincubation with an anti-hu- lymphocytes man immunoglobulin IgG preparation. In normals, 45–80% of peripheral blood lymphocytes formed rosettes, whereas patients with chronic lymphocytic leukemia and a patient with leukemic IgM-producing malignant lymphoma had an extremely reduced percentage of RFL (3–9%). In patients with Hodgkin’s disease, lymphocytic lymphosarcoma, ‘reticulum-cell sarcoma’, giant follicular lymphoma, Ig-producing lymphoma, and multiple myeloma, the proportion of RFL was either diminished or within the normal range (20–63%).