Liver is a prime site for conversion of inorganic arsenic (iAs) to methylated metabolites, including methylarsenicals (MAs) and dimethylarsenicals (DMAs). To assess interindividual variation in the capacity of liver to metabolize iAs, we examined the metabolic fate of arsenite (iAs(III)) in normal primary human hepatocytes obtained from eight donors and cultured under standard conditions. Methylation rates, yields, and distribution of arsenicals were determined for hepatocytes exposed to 0.3-30 nmol of iAs(III)/mg of protein for 24 h. Although the accumulation of arsenic (As) by cells was a linear function of the initial concentration of iAs(III) in culture, the concentration of As retained in cells varied several fold among donors. DMAs was the major methylated metabolite found in cultures exposed to low concentrations of iAs(III); at higher concentrations, MAs was always predominant. Maximal rates for methylation of iAs(III) were usually attained at 3 or 9 nmol of iAs(III)/mg of protein and varied about 7-fold among donors. For most donors, the methylation rate decreased at the highest iAs(III) concentrations. MAs was the major methylated metabolite retained in cells regardless of exposure level. DMAs was the major methylated metabolite found in medium. The interindividual differences in rates for iAs(III) methylation were not strictly associated with variations in basal mRNA levels for cyt19, an As-methyltransferase. Analysis of the coding sequence of cyt19 identified one heterozygote with Met287Thr mutation in a single allele. Thus, genetic polymorphism of cyt19 along with other cellular factors is likely responsible for interindividual differences in the capacity of primary human hepatocytes to retain and metabolize iAs(III).