We have investigated the possibility of measuring autoantibodies to IA-2 (IA-2A) using recombinant protein expressed in E. coli in a new radioassay. The intracellular part of IA-2 (IA-2ic) was expressed in E. coli as a biotinylated fusion protein and affinity-purified on a streptavidin column. The average yield of IA-2ic was about 1 mg purified protein from one litre of culture medium with E. coli. We could demonstrate the immunological activity of this material by blocking the autoantibody reactivity to in vitro synthesised IA-2ic. The IA-2ic fusion protein was then radiolabelled with 125I, purified by HPLC, and used in an immunoprecipitation assay for the detection of IA-2A. Sera from 46 of 68 (67%) patients with Type-I diabetes were positive by this radioassay, in contrst to only 2 of 50 (4%) patients with autoimmune thyroid disease and 1 of 114 (1 %) controls. There was a correlation between this radioassay and the previously established radioligand assay using synthesized 35S-methionine-labelled IA-2ic in vitro (r = 0.79, p < 0.001). We conclude that E. coli-derived IA-2 has the correct immunogenic conformation, and can be used for the detection of IA-2A with a similar sensitivity and specificity as the validated radioligand assay. This new assay can facilitate the measurement of IA-2A in routine laboratories where the radioligand assay is inconvenient or not available.