This immunohistochemical study describes the infiltration pattern of monocytes-macrophages and dendritic cells during the development of insulitis and diabetes in the NOD mouse. A panel of monoclonal antibodies (MoAbs) was used to analyze pancreases of nondiabetic (glucosuria negative) male and female NOD mice at 3, 7, 10, and 17 weeks of age. BALB/c female mice 17-weeks-old, diabetic NOD female mice 20- to 30-weeks-old, and nondiabetic NOD male mice 22-weeks-old were used as controls. Three MoAbs (viz., ER-MP23, MOMA1, and BM8) were special and appeared to identify macrophage/dendritic cell subsets that either had a characteristic infiltration pattern in the initial phases of the autoimmune reaction before T-cell infiltration or were typical for the later β-cell destructive insulitis process. 1) Raised numbers of ER-MP23⁺ and MOMA-1⁺ dendritic cells/macrophages were characteristic for the initial phases of the NOD insulitis in 3-week-old mice. The cells were found in and near swollen para-insular vessels. In 7-week-old mice, these ER-MP23⁺ and MOMA-1⁺ cells had accumulated around the islets and were the first hematopoietic cells detectable at these spots. 2) From 7 weeks of age onward, BM8+ macrophages could be found in the para- and peri-insulitis processes. However, only in females were these BM8+ macrophages found to infiltrate into the islets. In lymphoid tissues, ER-MP23 predominantly reacts with macrophages/dendritic cells present in the subcapsular and interfollicular sinuses of lymph nodes and the T-cell zones of these lymph nodes. ER-MP23 also reacts with tissue macrophages/dendritic cells. MOMA-1 reacts with the marginal metallophilic macrophages of the spleen and with sinus macrophages of the lymph node. Both populations of cells are likely to be involved in antigen presentation in lymphoid tissues as well as in the NOD peri-insulitis. BM8 in lymphoid tissues predominantly reacts with the phagocytosing macrophages present in the red pulp of the spleen. Because β-cell destruction and glucosuria almost exclusively take place in NOD females, our findings suggest that BM8⁺ macrophage infiltration into the female islets is linked to a β-cell destructive process, either as a destructive type of infiltration or as an infiltration meant to remove the β-cell debris caused by another immune assault.