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 ICA69 antibodies to patients with recent-onset IDDM and to patients with other immune-mediated diseases. ICA69 antibodies were determined by Western blot using an affinity purified recombinant fusion protein of ICA69 and maltose binding protein. ICA69 antibody quantities were determined as titres using a titration curve of a standard serum as reference. Mean logarithmic ICA69 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 ICA69 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 < 0.001). The percentage of sera with ICA69 antibody titres above the 2 SD level of normal subjects was 21% in IDDM, 31% in rheumatoid arthritis and 6% in healthy blood donors. None of the patients with autoimmune thyroid disease (n = 20), inflammatory bowel disease (n = 9) or multiple sclerosis (n = 7) had elevated ICA69 antibodies. In IDDM, presence of ICA69 antibodies persisted and the titre remained the same over 18 months of follow-up. The relationship of ICA69 antibodies to islet cell antibodies (ICA) or insulin autoantibodies (IAA) was tested. The production of ICA69 antibodies was not associated in diabetic patients with the presence of any of the two other autoantibodies.(ABSTRACT TRUNCATED AT 250 WORDS)