The stable fly, Stomoxys calcitrans (L.) (Diptera: Muscidae), is an economically important pest of livestock. Previous studies demonstrated lymphocyte suppression by crude salivary gland extract (SGE) of the stable fly. A dominant 27-kDa protein identified in the SGE was reported to stimulate immunodominant antibody responses in exposed cattle. The purpose of this study was to determine whether this protein, now identified as ahomolog of insect proteins named antigen 5 (Ag5), was responsible for the lymphocyte suppression and whether naive calves can mount an immune response to it. Calves raised in the winter were immunized with recombinant Ag5 (rAg5) expressed in Drosophila S2 cells or with "natural" Ag5 protein isolated by preparative gel electrophoresis of SGE. Control calves were immunized with adjuvant alone. Rising antibody concentrations to rAg5 were detected in two of three calves immunized with rAg5 and one of three calves immunized with natural Ag5. Recall lymphocyte responses to rAg5 were detected at 21 and 28 d postimmunization in calves immunized with rAg5 but not in calves immunized with the natural Ag5 or those exposed to adjuvant alone. Mitogen-stimulated bovine lymphocyte responses were not suppressed by rAg5. Further investigation using immunoblotting revealed that rAg5 binds to the Fc and F (ab')2 portions of bovine IgG, but not to an Fab fragment. These findings suggest that Ag5 of the stable fly salivary gland is not immunosuppressive but that it has immunoglobulin binding properties and can invoke specific antibody and memory lymphocyte responses in immunized calves.