The purpose of this study was to investigate effect of water extract of Carthami Flos on the proliferation of human cancer cell-lines. The effects of Carthami Flos on the proliferation of A431, HeLa, MOLT-4, K562 cells, Balb／c 3T3 cells, mouse thymocytes, splenocytes and human lymphocytes were estimated by MTT colorimetric assay. The results were as follows; 1. Carthami Flos did not effect A431, HeLa, MOLT-4, K562 cells. 2. The cytotoxicity of mitomycin C on K562 cells was increased by the combination of Carthami Flos. 3. Carthami Flos inhibited the proliferation of Balb／c 3T3 cells. 4. Carthami Flos stimulated the proliferation of thymocytes. 5. Carthami Flos stimulated the proliferation of splenocytes. 6. Carthami Flos stimulated the proliferation of human lymphocytes.