The lysosomotropic compound L-leucine methyl ester (Leu-OMe) was utilized to delineate the phenotype of the accessory cells involved in human B and T cell activation in vitro. Leu-OMe was shown to cause lysosomal disruption and selective death of human monocytes (M phi). After 30-45 minute incubations with this agent, human peripheral blood mononuclear cells (PBM) were nearly completely depleted of M phi. Associated with this M phi depletion, PBM were rendered unresponsive to a variety of T and B cell mitogens including the plant lectins phytohemagglutinin, concanavalin A, and pokeweed mitogen as well as the oxidative mitogens sodium periodate and neuraminidase plus galactose oxidase. Leu-OMe mediated loss of responsiveness was the result of a selective loss of an accessory cell necessary for each of these responses since reconstitution was accomplished by the addition of a M phi-enriched adherent cell population VSports手机版. While intact adherent cells could reconstitute responsiveness, crude M phi supernatants or highly purified human IL 1 alone were ineffective. Further identification of the Leu-OMe sensitive accessory cell indicated that it was entirely contained within the fraction of the adherent population identified by the monoclonal anti-M phi antibody, 63D3. The mechanism by which Leu-OMe Killed M phi was dependent on the lysosomal content of these cells, since agents that altered lysosomal enzyme activity such as chloroquine or NH4Cl protected M phi from Leu-OMe. Thus, the selective killing of M phi by Leu-OMe appeared to relate to the characteristically rich endowment of lysosomes within these cells. These results support the conclusion that a lysosome-rich, leucine methyl ester-sensitive, intact M phi identified by the monoclonal anti-M phi antibody 63D3 is the circulating accessory cell required for mitogen-triggered human B and T cell activation. .