Sterigmatocystin bears a close structural relationship to aflatoxin $B_1$ and is a carcinogenic compound that has been shown to affect various species of experimental animals. Reaction and toxicity of sterigmatocystin in the artificial gastric juice were investigated. Sterigmatocystin was degraded in artificial gastric juice and extracted by the method of A.O.A.C. After cleaned up by silica gel column chromatography, this substance was detected and characterized by thin layer chromatography, UV, IR and mass spectra. It showed $R{mathcal{f}}$ 0.4 and brick-red color by TLC. Especially, in the mass spectrum of it, fragment peak at m/e 327 was due to the loss of the $-CH_3$ and $-H_2O$, fragment peak at m/e 341 was due to the loss of the $H_2O$ and $-H^+$, and fragment peak at m/e 239 was due to the loss of the 2-chloro-tetrahydrofuran and methyl group from the parent molecule. Therefore, a degraded substance of sterigmatocystin reacted in artificial gastric juice (Sub. K) was estimated with additional formation of hydrochloric acid. In four-day-old chicken embryos, the mean lethal dose $(LD_{50})$ was $140;{mu}g/egg$, and 90 to 100% of the embryos were killed with 1 mg/egg. This $LD_{50}$ $140;{mu}g/egg$ compared with an $LD_{50}$ $14.69;{mu}g/egg$ for sterigmatocystin (acute toxicity) showed the substance to be much less toxic than sterigmatocystin.