The purpose of this study was to examine the nature of the blood-brain barrier in experimental brain metastases. Syngeneic fibrosarcoma or melanoma cells were injected into the internal carotid arteries of mice. Several weeks later, once the experimental brain metastases were established, the mice were given injections iv of sodium fluorescein. The capillaries within the metastatic foci were enlarged and irregular, but there was no leakage of sodium fluorescein, showing that the blood-brain barrier was intact. The neoplastic lesions were infiltrated by mononuclear phagocytes, which were identified by immunohistochemical localization of the macrophage-specific antigen F4/80, class II major histocompatibility complex (MHC) antigens, and the macrophage product interleukin-1 (IL-1). The metastatic foci contained numerous stellate macrophages that expressed F4/80 and MHC class II antigens, but little IL-1. Round, monocyte-like F4/80 and MHC class II-positive cells were also observed within the tumor lesions and adhering to walls of the tumor microvasculature. Mice with fibrosarcoma brain metastases also had edematous lesions at sites remote from the metastatic foci that contained numerous astrocytes expressing class II MHC but not F4/80 antigens. In conclusion, the blood-brain barrier is intact within experimental brain metastases, yet macrophages of blood monocyte origin can infiltrate the lesions.