In an effort to identify membrane components participating in coupling stimulus to secretion in mast cells, monoclonal antibodies were produced from spleen cells of mice immunized with plasma membranes isolated from rat mast cells of the RBL-2H3 line. The resultant mAbs were screened by their capacity to modulate the secretory response of these cells to crosslinking of their type 1 Fc epsilon receptor (Fc epsilon RI). Following this scheme, we obtained a hybridoma designated B17, which secretes an IgM-class mAb (B17) that binds to and modulates secretion from RBL-2H3 cells. By immunoblotting, B17 was shown to bind to a membrane component of low molecular weight, later identified as a glycolipid. While B17 partially inhibits IgE binding to RBL-2H3 cells, no noticeable inhibition of B17 binding by IgE was observed. mAb B17 does not cause any secretory response on its own, and its modulatory effect on Fc epsilon RI-mediated secretion is bimodal: it either enhances or inhibits secretion, depending on the B17 dose and also on the nature and dose of the agent used for crosslinking the Fc epsilon RI. When secretion was induced by IgE and suboptimal or optimal doses of multivalent antigen, B17 (2-80 nM) caused an increase in secretion. However, higher doses of B17 (greater than 150 nM) inhibited secretion. Secretion induced by supraoptimal doses of antigen, or by the Fc epsilon RI-specific mAb F4 was inhibited by B17 at all the dose range tested (2-200 nM). In contrast, B17 had no effect on secretion induced by Ca2+ ionophores. These results demonstrate that Fc epsilon RI function is modulated by a mAb binding to a membrane glycolipid.