OBJECTIVE--To search for possible serum factors (immunochemical abnormalities) that reflect HLA-B27 associated inflammatory process with the proliferative endarteritis, which is an important cause of severe bradycardia and aortic valve regurgitation. PATIENTS AND METHODS--Seventy four men with pacemakers were studied: 24 were HLA-B27 positive and had associated rheumatic and cardiac disorders, 13 were B27 positive but had no clinical or radiographic signs of a related rheumatic condition, and 37 were B27 negative controls. Randomly obtained serum samples were examined for a series of serum factors. RESULTS--Thirteen (57%) of the 23 patients with HLA-B27 and associated rheumatic and cardiac conditions had platelet aggregating activity in their serum. No such activity was found in sera from patients in the other groups. None the less, immunochemical abnormalities were common among patients of all groups; 30 (41%) had antinuclear antibodies or rheumatoid factor or both. CONCLUSION--The platelet aggregating activity found in patients with HLA-B27 and associated rheumatic and cardiac conditions may reflect serum factors that increase the stickiness of platelets and increase their adhesion to the vessel wall. This suggests a link via release of platelet derived growth factor(s) with the characteristic histopathological feature of proliferative endarteritis. Immunochemical abnormalities were common in serum from all men with pacemakers.