Human T lymphocyte subpopulations can be identified on the basis of surface receptors for G or M immunoglobulins (TM and TG cells respectively). Isolated TM or TG cells can be analyzed in vitro for their morphology and their functional properties. In addition to unique characteristics observed in light and electron microscopy, TM and TG cells show a different pattern of responsiveness to PHA and alloantigens. The major functional difference so far observed concerns their interaction with B lymphocytes. TM cells provide help for the T-dependent responses of B lymphocytes to pokeweek mitogen (PWM), whereas TG cells suppress B lympohcyte activation in this in vitro system by inhibiting helper TM cells. Imbalances and/or functional alterations of TM and TG cells are present in various immunological disorders and are possibly involved in the pathogenesis of these diseases.