Cytoplasmic islet cell antibodies (ICAs) are the classical serological markers for diagnosis and prediction of IDDM, but high technical demands have limited the widespread use of the histochemical ICA test. To investigate whether combined analysis of autoantibodies to two defined islet antigens can replace the histochemical ICA test, we established quantitative radioimmunoassays for autoantibodies to glutamate decarboxylase (GAD65-A), the tyrosine phosphatase IA2/ICA512 (IA2-A), and the cytoplasmic part of IA2 (IA2c-A). The GAD65-A and IA2c-A profiles of 920 sera from healthy individuals and from patients with IDDM, other organ-specific autoimmune diseases, and polyendocrine autoimmune syndrome were compared with the ICA profiles from these same individuals. Combined analysis of GAD65-A and IA2c-A detected 93-100% of the ICA+ sera, and, at equal specificity, improved the diagnostic sensitivity (85%) for IDDM compared with that of ICA (74%). This effect was especially pronounced in children with disease onset before 16 years of age (91% sensitivity). To replace ICA testing in risk assessment for IDDM, we designed a strategy adapted to study groups with low antibody prevalence. A combined radioimmunoassay for single-step detection of GAD65-A and IA2c-A was developed, and positive sera were reanalyzed to define their single autoantibody specificity. We identified 93% of the ICA+ sera from 204 first-degree relatives of IDDM patients. Single-step detection reduced costs and effort by more than 40% compared with separate testing, allowing an efficient large-scale screening of sera for GAD65-A and IA2c-A in IDDM. In sum, GAD65-A and IA2c-A detected much ICA reactivity, and their combined evaluation and detection is suitable to replace the histochemical ICA test.