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Islet cell antigen (ICA) 69 is a newly-recognized islet cell antigen to which autoantibodies have been observed in prediabetic relatives of patients with insulin-dependent-diabetes mellitus (IDDM). Here we extend the earlier analysis of ICA 69 antibodies to patients with recent-onset IDDM and to patients with other immune-mediated diseases. ICA 69 antibodies were determined by Western blot using an affinity purified recombinant fusion protein of ICA 69 and maltose binding protein. ICA 69 antibody quantities were determined as titres using a titration curve of a standard serum as reference. Mean logarithmic ICA 69 antibody titres were 3.4 (±1.4) in 99 patients with acute IDDM compared to 2.8 (±0.9) in 49 healthy blood donors (p<0.001). A higher mean ICA 69 antibody titre of 4.1 (±0.8) was observed in 16 patients with rheumatoid arthritis in comparison to acute IDDM (p<0.01) and healthy control subjects (p …