Target antigens defined by autoantibodies in IDDM include insulin, a putative glycolipid that reacts with islet cell antibodies, and a 64,000-M(r) protein recently identified as glutamic acid decarboxylase. In addition, some IDDM sera that contain antibodies to glutamic acid decarboxylase also coprecipitate a 38,000-M(r) protein from islets. This study used a high titer anti-38,000-M(r) serum to screen bacteriophage lambda cDNA expression libraries and identified human islet and placental clones encoding jun-B, the nuclear transcription protein, of predicted 38,000 M(r). Peripheral blood T-cells exhibited significant proliferation in response to a recombinant fragment of jun-B (amino acids 1-180) in 12 of 17 (71%) recent-onset IDDM subjects, 8 of 16 (50%) ICA-positive first-degree relatives of IDDM subjects who were at risk, 3 of 12 (25%) other autoimmune disease subjects, and 0 of 10 healthy control subjects. Proliferation to tetanus toxoid did not differ significantly between the groups. Responses to jun-B were not related to age, sex, or human leukocyte antigen status. Thus, autoreactive T-cells identify a novel antigen, p38 jun-B, in IDDM and appear to indicate subjects at risk for the development of clinical disease.