The effect of salivary gland extract of the stable fly, Stomoxys calcitrans (L.), on bovine lymphocyte proliferation was determined, and antibody reactivity to salivary gland proteins was characterized in cattle exposed to stable flies. Salivary glands were dissected from male and female flies (4–8 d after eclosion), and protein extracts were made by freeze–thaw cycles. Salivary gland extract (SGE, 1 and 5 μg) significantly inhibited mitogen-driven proliferation of bovine lymphocytes, compared with 1 and 5 μg of identically prepared midgut extract (ANOVA, P < 0.05). Phytohemagglutinin A (PHA) stimulated lymphocyte responses were suppressed by 61.7 and 79.5% (mean values) with 1 and 5 μg of SGE, whereas concanvalin A (Con A) stimulated responses were suppressed by 62.9 and 77.1% (1 and 5 μg). In contrast, midgut extract (1 and 5 μg) minimally suppressed PHA (12.7% ± 12.6 and 18.7% ± 15.5) and Con A-driven responses (13.8% ± 20.5 and 24.6% ± 14.9), respectively. Viability studies using propidium iodide and flow cytometry demonstrated that SGE was not cytotoxic. Two-color immunofluorescence studies identified T and B lymphocytes as the nonviable cells in the cultures. Western blot analysis of serum collected from five dairy cows during periods of low and high fly exposure identified an immunodominant 27 kDa protein among the salivary gland proteins. These results indicate that exposure of cattle to stable fly saliva during blood feeding results in an antibody response to salivary proteins and that the saliva has a potential to modulate T lymphocyte function.