Circulating platelets from 15 patients with primary Sjögren's syndrome (primary SS) and from 15 normal controls were enumerated and their aggregability determined. Blood platelet concentrations were within the normal range, except for two patients in whom they were slightly decreased. Platelet aggregation was enhanced in patients, when measured upon stimulation with epinephrine (p less than 0.05), ADP (p less than 0.01) and collagen (p less than 0.01). Plasma and saliva from 17 patients with primary SS and from 11 normal controls were examined for the platelet-specific release product beta-thromboglobulin (beta-TG). P-beta-TG was increased in the patients, although not significantly (p greater than 0.05). In saliva beta-TG was detected in 5 patients (11-150 ng/ml), but not in any of the controls. There was no correlation between levels of beta-TG in plasma and saliva. Lower lip minor salivary glands from 17 patients under evaluation for SS were examined for platelet accumulation. Indirect immuno-peroxidase staining with the monoclonal mouse anti-human platelet glycoprotein Ib antibody, showed platelet accumulation intravascularly in the inflamed areas. The combined results are in accordance with the hypothesis that platelet activation occurs in the salivary glands of patients with primary SS.