Ly-6C is a differentiation antigen that distinguishes T-lymphocyte subsets. In concordance with previous results, splenocytes from NOD mice do not express the epitope recognized by anti-Ly-6C monoclonal antibodies (MoAbs), including MoAb HK1.4 in this study, and cannot be stimulated to proliferate in response to HK1.4. However, when splenocytes from NOD mice were stimulated in vitro with the anti-CD3 MoAb 145-2C11, T lymphocytes expressing Ly-6C were detected after 48 h of stimulation, with as many as 25% of lymphocytes expressing this antigen with prolonged passage in culture. Most of the cells expressing Ly-6C were Thy-1.2+, CD4+, and CD8- and proliferated after stimulation with HK1.4. To further understand the failure of NOD splenocytes to express Ly-6C, freshly isolated cells were stimulated with alpha/beta-interferon (IFN-alpha/beta) and IFN-gamma. Although these lymphokines induced expression of Ly-6A and Ly-6C in splenocytes from C57BL/6J mice and Ly-6A in NOD cells, Ly-6C was not induced on NOD cells. Because Ly-6C expression on splenocytes was a marker of activation via the CD3 T-lymphocyte receptor complex, we also examined expression of Ly-6C on T lymphocytes within islets showing insulitis in vivo. Lymphocytes that were Ly-6C+ were identified within islets on histological sections of pancreas, whereas Ly-6C+ cells in the spleen from the same mouse could not be detected. Our findings imply functional abnormality in expression of Ly-6C in NOD mice.(ABSTRACT TRUNCATED AT 250 WORDS)