The aim of this study was to observe modifications occurring within the blood vessels of pancreatic islets in low-dose streptozocin (STZ)-treated mice. Forty-two male C57BL/6J mice were given 40 mg STZ on 5 consecutive days and killed 6 or 10 d after the first STZ injection. Immunocytochemistry revealed a dense accumulation of cells carrying the 7D10 epitope, related to macrophage inhibitor factor, arranged around the 'vascular pole' of the islets on d 6, and a small number of positive cells within the islets on d 10. Many Ia positive cells (equivalent to MHC class II antigen positive elements) were also found at the periphery of the islet. Ultrastructural observations showed the presence of monocytes within almost all the capillary vessels encircling the islets and entering them at the 'vascular pole' (d 6). On d 10 capillary lumina no longer contained marginating monocytes but activated macrophages were seen in the islet parenchyma.