Incubation of human elutriator-purified monocytes with anti-HLA-DR or DQ antibody inhibited the release of arachidonic acid induced by serum- treated zymosan (STZ), a phagocytic stimulus which is known to induce inositol phospholipid hydrolysis and Ca=2 influx. However, only anti- HLA-Dr antibody partially inhibited STZ-induced inositol phosphate hydrolysis and concanavalin-A- induced Ca+2 influx. Incubation with anti-HLA-Dr or DQ antibody inhibited phorbol ester-induced AA release as well as superoxide production and Il-1 release. Inhibition of monocyte function by anti-class II antibodies was not accompanied by cAMP elevation. Furthermore, addition of exogenous db-cAMP and other agents (forskolin, cholera toxin, or IBMX) that can increase cAMP levels through different mechanisms, alone or in combination with anti-HLA antibodies, had no inhibitory effect on factor release. Our results demonstrate that perturbation of class II molecules down modulates cell activation at more than one point of the signal transduction pathway with dominant inhibition distal to inositol phosphate hydrolysis. They also suggest that the inhibition by anti-HLA class II antibody is probably not mediated via cAMP elevation.