Aims/hypothesis. Islet cell autoantibodies are a specific marker for Type 1 (insulin-dependent) diabetes mellitus. Standardisation of islet cell antibodies and the uniform reporting in International units is critical to research and the development of assays for islet cell autoantibodies as diagnostics.¶Methods. The suitability of a candidate serum to serve as the international standard for islet cell antibodies was studied by 19 participants in 8 countries. In addition, the purpose was to investigate whether the serum could also serve as a standard for antibodies to the 65 000 M_r isoform of glutamic acid decarboxylase (GAD65) and islet antigen-2 (IA-2). Control sera were included in the study to assess the validity of the various assay systems. The sera were lyophilized to World Health Organization criteria and the candidate serum assigned the ampoule code number 97/550.¶Results. The use of 97/550 was shown to notably reduce inter laboratory variability in the measurement of islet cell antibodies. In addition, there was a pronounced reduction in inter laboratory variability in the measurement of GAD65 and IA-2 antibodies. ¶Conclusions/interpretation. On the basis of the results reported here and with agreement of the participants, the preparation 97/550 has been established by the World Health Organization Expert Committee on Biological Standards for establishment as the first international standard for islet cell antibodies, with an assigned potency of 20 international units. In addition, 97/550 can serve as an international reference reagent for specific GAD65 antibodies, with an assigned potency of 100 units. It can also serve as a National Institute of Biological Standards and Control (NIBSC) reference reagent for IA-2 antibodies for evaluation of assays for this material. [Diabetologia (2000) 43: 1282–1292]