Most of the enzymes used in routine biochemical tests have been detected in circulating enzyme-linked immunoglobulin (Ig) complexes (E-Ig). The complexes often cause unexplained hyperenzymemia and give anomalous patterns on isozyme electrophoresis. However, E-Ig does not appear to be associated with any pathological conditions. The present study was undertaken to determine the clinical characteristics of patients with E-Ig. More than 42,000 patients selected at random and 10,000 blood donors were screened for lactate dehydrogenase (LDH)-linked Ig complexes (LDH-Ig), amylase (Amy)-linked Ig complexes (Amy-Ig), alkaline phosphatase (AP)-linked Ig complexes (AP-Ig) and creatine kinase (CK)-linked Ig complexes (CK-Ig). LDH-Ig and Amy-Ig were screened by electrophoresis for routine isozyme analysis and identified by precipitin reactions. AP-Ig and CK-Ig were screened and identified by counter immunoelectrophoresis. The incidence of all E-Ig ranged from 0.18% to 0.61% in patients and from 0.03% to 0.23% in blood donors. The incidence curves of E-Ig were age-related, generally increasing with age. The curves of E-IgG's for different enzymes resembled each other in shape but those of E-IgA's did not. A positive correlation between E-IgG and autoimmune diseases was present. Consequently, it is concluded that both autoimmune diseases and age are the common clinical factors involved in E-IgG. No specific diseases were found for cases with E-IgA. However, it is suggested that some CK-IgA, but not all, is caused by elevated serum CK activity.