Leu 7+ cells in germinal centers of lymphoid tissues largely (greater than 90%) coexpress the T helper-cell marker, Leu 3. In this study we have isolated Leu 7+ (Leu 3+) cells from pharyngeal and palatine tonsils and we have analyzed their surface phenotype, morphologic and cytochemical characteristics, and functional properties. All of these features have been compared with those of T helper-cell populations with natural killer (NK)-like characteristics that we have previously described in peripheral blood. Leu 7+ (Leu 3+) cells from tonsil germinal centers display morphological and cytochemical features of granular lymphocytes and express the T3 marker in the absence of Leu 15. Following stimulation with phytohemagglutinin (PHA) or anti-T3, Leu 7+ (Leu 3+) cells express interleukin-2 (IL-2) receptors and proliferate to some extent in response to IL-2. The localization of Leu 7+ (Leu 3+) cells in B-dependent areas of lymphoid tissues suggests that they may play a regulatory role in B-cell proliferation and/or differentiation. Here we show that Leu 7+ (Leu 3+) cells do not produce B-cell growth factor (BCGF) and do not help pokeweed mitogen (PWM)-driven B-cell differentiation. Therefore, Leu 7+ (Leu 3+) germinal-center cells are distinct from "classic" T-helper cells of blood and lymphoid tissues.