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Monomethylarsonous acid (MMA^III) has been detected for the first time in the urine of some humans exposed to inorganic arsenic in their drinking water. Our experiments have dealt with subjects in Romania who have been exposed to 2.8, 29, 84, or 161 μg of As/L in their drinking water. In the latter two groups, MMA^III was 11 and 7% of the urinary arsenic while the monomethylarsonic acid (MMA^V) was 14 and 13%, respectively. Of our 58 subjects, 17% had MMA^III in their urine. MMA^III was not found in urine of any members of the group with the lowest level of As exposure. If the lowest-level As exposure group is excluded, 23% of our subjects had MMA^III in their urine. Our results indicate that (a) future studies concerning urinary arsenic profiles of arsenic-exposed humans must determine MMA^III concentrations, (b) previous studies of urinary profiles dealing with humans exposed to arsenic need to be re-examined and re-evaluated, and (c) since MMA^III is more toxic than inorganic arsenite, a re-examination is needed of the two hypotheses which hold that methylation is a detoxication process for inorganic arsenite and that inorganic arsenite is the major cause of the toxicity and carcinogenicity of inorganic arsenic.