During subarachnoid hemorrhage (SAH), coagulated blood in the subarachnoid space may be regarded as foreign by the immune system. To investigate how cerebral arteries are affected by activation of the host immune system, foreign body, polystyrene latex beads were injected into the cerebrospinal fluid (CSF) space of rabbits, and the caliber changes of the basilar arteries were studied for 7 days by angiography. Prolonged arterial narrowing peaking on day 2 was observed after cisternal injection of the beads. The increase in peak narrowing correlated with an increase in the number of beads injected. The course of the change in vessel caliber over 7 days was similar to that seen in cerebral vasospasm caused by SAH. Also investigated was the preventive effect of the synthetic serine protease inhibitor, FUT-175 on the arterial narrowing caused by the cisternal injection of the latex beads. The administration of FUT-175 significantly prevented latex beads-induced vasospasm (p < 0.01). The possible role of a non-specific immune response is discussed, and also the role of the serine protease cascades in the development of cerebral vasospasm.