The novel coinhibitory receptor BTLA may have a regulatory role in maintaining peripheral tolerance; however, its role in autoimmune diabetes is unknown. In this study, we show that anti-BTLA mAb 6F7 selectively depleted pathogenic B and CD4+ T(H) cells; enhanced the proportion of cells with the forkhead box p3+ PD-1+CD4+ regulatory T phenotype; and increased the production of potentially protective (IL-10) and detrimental (IL-2, IFN-gamma) cytokines in NOD mice. As interactions between BTLA and PD-1 coinhibitory pathways have been described in the cardiac allograft model, we also investigated if these pathways may have significant interaction in autoimmune diabetes. Anti-BTLA inhibited anti-PD-1-potentiated total IL-12 (p40+p70) production, suggesting the possibility that anti-BTLA may have a greater effect in the setting of anti-PD-1-triggered diabetes. To test this, NOD mice at 4 and 10 weeks of age were treated with anti-BTLA mAb, anti-PD-1 mAb, both mAb, or isotype control and were monitored for diabetes development. Although anti-BTLA mAb delayed diabetes onset significantly in 10- but not 4-week-old NOD mice, anti-BTLA mAb attenuated anti-PD-1-induced diabetes in both age groups. Hence, strategies targeting BTLA+ lymphocytes or therapies enhancing the BTLA-negative cosignal may prove valuable in treating autoimmune diabetes.