Cyanobacterial toxins, microcystins, are very potent hepatotoxins and their occurrence has been reported all over the world. They could threaten human health when toxic Microcystis occurs in water supply reservoirs. In this study, the effects of several environmental factors on production and degradation of toxins produced by cyanobacteria in Lake Soyang have been studied. A new rapid quantification method of microcystins, using fluorescence for a detection signal and a lateral-flow-type immunochromatography as a separation system, was used. Chlorine, potassium permanganate, and hydrogen peroxide were used as chemical oxidants for the degradation of microcystin LR. When chlorine was used, the efficiency of degradation was the highest. The degradation reaction took 40 minutes.