Chlorine gas was used to treat aflatoxin B1 (AFB1). The time-related exposure study showed that 4 ml (15 mg) pure chlorine gas caused about 90% destruction of 100 micrograms AFB1 within 10 min, at standard temperature and pressure. Four fluorescent reaction products were produced, two of which were identified as 8,9-dichloro-AFB1 and 8,9-dihydroxy-AFB1 (diol). The use of [14C]AFB1 confirmed the 90% destruction of the compound by chlorine gas. An increased destruction of AFB1 also occurred when an increased amount of chlorine gas was used. The mutagenic activity of the AFB1 sample treated for 10 min was reduced to about 5% of the untreated control using the Salmonella typhimurium strain TA98 in the presence of a rat-liver S-9 mix. A similar time-related reduction in AFB1 toxicity after chlorine treatment was also achieved using the chicken embryo toxicity assay.