✓ In order to clarify the possible role of immunological reaction in the pathogenesis of cerebral vasospasm, the authors examined the prophylactic effect of the immunosuppressant agents FK-506 and cyclosporin A on chronic vasospasm in a canine two-hemorrhage model. While a mean constriction of the basilar artery to 81.0% ± 4.0% (± standard error of the mean) occurred on Day 2 and to 63.8% ± 3.5% on Day 7 in the untreated group, constriction to 77.9% ± 3.4% on Day 2 and 62.8% ± 3.0% on Day 7 was demonstrated in the FK-506-treated group (difference not significant). This tendency was also noted in the cyclosporin A-treated group, with basilar artery constriction to 81.8% ± 3.7% and 56.3% ± 2.7%, respectively (difference not significant). The histological changes of the basilar artery, including corrugation of the elastic lamina, detachment of endothelial cells, and vacuolar formation in the smooth-muscle layer were not different in the two treated groups and the one control group. Since these immunosuppressant agents are known to inhibit the release of interleukin-2 (IL-2), the level of IL-2 was examined in the cerebrospinal fluid of patients with cerebral vasospasm. While interleukin-1 gradually increased in level as time passed, the level of IL-2 was consistently low during the course of the study, indicating less participation of IL-2 in the pathogenesis of cerebral vasospasm. This clinical observation matched the experimental results. The authors conclude that cell-mediated immunoreaction, initiated mainly by IL-2, plays little role in the pathogenesis of cerebral vasospasm.