An antivitamin B₆, 4'-methoxypyridoxine (1) was isolated from the seed of Ginkgo biloba L. (Ginkgoaceae). The albumen of the seed of G. biloba L., which is called "Gin-nan or Ginkyo", is used as a crude drug in China and as food in Japan. However, in Japan, there have been about 70 reports of "Gin-nan sitotoxism." In this paper, we report that the substance responsible for this sitotoxism is 4'-methoxypyridoxine (1), which is known to have antivitamin B₆ activities. This compound (1) is reported for the first time from natural products.