We have found that circulating human T lymphocytes are motile, whereas unstimulated B lymphocytes are not (RES, J.Reticuloendothel.Soc.20:331, 1976). Since motility is a functional morphologic marker of normal circulating T cells, we have begun to examine whether motility is a marker for T cells in immunologic disorders. Patients with CLL, a B cell malignancy, and x-linked hypc-γ-globulin-emia, a genetic B cell deficiency, were studied. Motile lymphocytes were determined by interference contrast microscopy. Blood lymphocytes were examined in the absence of plasma or serum since these inhibit lymphocyte motility. Circulating T cells were quantitated by E-rosetting and B cells by EAC-roserting or immunofluorescent detection of surface immunoglobulins. E-rosetting lymphocytes were decreased in CLL and normal in hypo-γ-globulinemia. Immunoglobulin bearing lymphocytes were greatly increased in CLL and were decreased in hypo-γ-globulinemia. Motile lymphocytes (normal x̄ ± SD, 45±5) were decreased in CLL (11 ± 3), but were normal in hypo-γ-globulinemia (56 ± 13). The number of motile lymphocytes in these two disorders correlated quantitatively with the number of T lymphocytes as determined by E-roserting. Thus, it appears that lymphocyte motility may be an important marker of T lymphocytes in certain disease states as well as in healthy individuals.