The Thy-1 antigen expressed on the surface of mouse T lymphocytes has been previously found to be involved in T cell activation. In the present study we have employed the anti-Thy-1.1 monoclonal antibody MRC OX7 and analyzed the expression and properties of Thy-1 antigen on the surface of peritoneal and pleural rat mast cells. Direct radioantibody binding assays, indirect immunofluorescence studies and flow cytometry analysis revealed that isolated rat mast cells express on their surface large amounts of the Thy-1 antigen. Scatchard analysis of the binding data indicated that at least one million Thy-1 molecules are expressed per cell. Immunoprecipitation studies carried out on mast cells demonstrated that MRC OX7 antibody recognizes a surface molecule of approximately 25 kDa that appears to correspond to Thy-1 antigen, and an additional molecule of 50 kDa. Incubation of the isolated mast cells with various concentrations of anti-Thy-1.1 antibody induced a rapid and concentration-dependent increase in the intracellular free calcium concentration ([Ca2+]i). The early increase in [Ca2+]i was observed in both Ca2+-supplemented and Ca2+-free media. This indicated that the initial increase in [Ca2+]i is due to a release of Ca2+ from internal stores. The [Ca2+]i increase was followed by an increase in the histamine release which was also dependent on antibody concentration. These data suggest that Thy-1 may act as an activation receptor on mast cells, analogously to its receptor function on T cells.