To test the hypothesis that oxidative stress is involved in inducing freezing injury of plants, nitroblue tetrazolium (NBT) was infiltrated into cabbage leaf tissues before they were frozen. NBT was reduced to formazans in freeze-thaw cycles. The ethanol-extractable mono-formazan was measured by a spectrophotometer, and used as a quantitative indicator of oxidative stress that developed during a freeze-thaw cycle. NBT per se as an oxidant increased the freezing injury slightly. When Me2SO as a cryoprotectant was infiltrated into leaf tissue, it apparently enhanced the freezing tolerance, and this is correlated to its ability in ameliorating oxidative stress, as indicated by the partial inhibition of NBT reduction. The results supported the hypothesis that freezing injury of plants is related to oxidative stress. The experiments indicated that it was possible that the reductant of NBT in the freeze-thaw cycle was superoxide anion free radical.