The T lymphocytes mediating autoimmune destruction of pancreatic beta cells in the nonobese diabetic (NOD) mouse model of insulin-dependent diabetes mellitus (IDDM) may be generated due to functional defects in hematopoietically derived antigen-presenting cells (APC). However, it has not been clear which particular subpopulations of APC (B lymphocytes, macrophages, and dendritic cells) contribute to the development and activation of diabetogenic T cells in NOD mice. In the current study we utilized a functionally inactivated immunoglobulin (Ig) mu allele (Ig mu null) to generate a "speed congenic" stock of B lymphocyte-deficient NOD mice that are fixed for linkage markers delineating previously identified diabetes susceptibility (Idd) genes. These B lymphocyte NOD.Ig mu null mice had normal numbers of T cells but were free of overt IDDM and insulitis resistant, while the frequency of disease in the B lymphocyte intact segregants was equivalent to that of standard NOD mice in our colony. Thus, B lymphocytes play a heretofore unrecognized role that is essential for the initial development and/or activation of beta cell autoreactive T cells in NOD mice.