The roles of adherent and nonadherent T cells in the responses to phytohemagglutinin (PHA) and concanavalin A (Con A) were studied. [3H]thymidine uptake by mononuclear cells from human thymus and peripheral blood was used to measure the relative influence of monocytes and B cells on the responses to these mitogens. The response to Con A was dependent on two populations of T cells, one adherent to cotton or nylon and the other nonadherent. The first population appeared to potentiate the latter. In contrast, the response to PHA was dependent only on the nonadherent T cell subpopulation. The responses to PHA and to Con A were both dependent on monocytes to approximately the same extent, but the response to Con A, unlike the response to PHA, was also slightly dependent on B cells.

The role of monocytes, B cells, and adherent and nonadherent T cells in the response of purified protein derivative of tuberculin (PPD) as measured by [3H]thymidine uptake in vitro has been explored. The response to PPD was found to be highly dependent on monocytes, to approximately the same extent as previously found for the responses to concanavalin A (Con A) and phytohemagglutinin. The response to PPD was also found to be highly dependent on a population of adherent T cells different from the adherent T cell population involved in the response to Con A. In the case of PPD, the effect was additive, as opposed to the potentiating effect seen for Con A. Further, the adherent T cells involved in the response to PPD were much more sensitive to hypotonic shock than those in the response to Con A. B cells were also found to be important in the response to PPD, although only to a slight extent.