In this study, we focused upon the immunologic aspects of Vogt-Koyanagi-Harada disease (VKH) by comparing the cytotoxic activity of peripheral blood leukocytes (PBL) to that of cerebrospinal fluid leukocytes (CSFL) against the human melanoma cell line (P-36) and the human cervical carcinoma cell line (HeLa-S3). The PBL from patients with VKH showed significant cytotoxic activity against P-36 (P less than 0.01), but did not show cytotoxic activity against HeLa-S3. The CSFL showed significantly weaker cytotoxic activity against P-36 compared to that of PBL (P less than 0.02). We also analyzed the cell membrane surface markers applying monoclonal antibodies on PBL and CSFL. The percentage of OKT8+ (CD8: T cytotoxic/suppressor lymphocytes) cells was significantly lower in CSFL than in PBL (P less than 0.05). There was a tendency toward a higher percentage of HLA-DR+ cells (B lymphocytes, monocytes, macrophages, and activated T lymphocytes) and a higher ratio of OKT4+/8+ cells (CD4/CD8: T helper/inducer lymphocytes/T cytotoxic/suppressor lymphocytes) in CSFL from patients with VKH than in their PBL (P less than 0.1).