Objective Sjögren's syndrome (SS) is an autoimmune condition affecting salivary glands, for which a clearly defined pathogenic autoantibody has yet to be identified. Autoantibodies that bind to the muscarinic M3 receptors (M3R), which regulate fluid secretion in salivary glands, have been proposed in this context. However, there are no previous data that directly show antisecretory activity. This study was undertaken to investigate and characterize the antisecretory activity of anti-M3R. Methods Microfluorimetric Ca2+ imaging and patch clamp electrophysiologic techniques were used to measure the secretagogue-evoked increase in [Ca2+]i and consequent activation of Ca2+-dependent ion channels in individual mouse and human submandibular acinar cells. Together, these techniques form a sensitive bioassay that was used to determine whether IgG isolated from patients with primary SS and from control subjects has antisecretory activity. Results IgG (2 mg/ml) from patients with primary SS reduced the carbachol-evoked increase in [Ca2+]i in both mouse and human acinar cells by ∼50%. IgG from control subjects had no effect on the Ca2+ signal. Furthermore, the inhibitory action of primary SS patient IgG on the Ca2+ signal was acutely reversible. We repeated our observations using rabbit serum containing antibodies raised against the second extracellular loop of M3R and found an identical pattern of acutely reversible inhibition. Anti-M3R–positive serum had no effect on Ca2+-dependent ion channel activation evoked by the direct intracellular infusion of inositol 1,4,5-triphosphate. Conclusion These observations show for the first time that IgG from patients with primary SS contains autoantibodies capable of damaging saliva production and contributing to xerostomia. The unusual but not unprecedented acute reversibility of the effects of anti-M3 autoantibodies is the subject of further research.