The sodium salt of 2,3-dimercaptopropane-1-sulfonic acid (Dimaval; DMPS) challenge test has been given previously to humans exposed to elemental mercury (vapor) or mercuric salts, but not mercurous salts. The test (300 mg p.o., after an 11-hr fast) was given to 11 factory workers who make a skin lotion that contains mercurous chloride, eight users of the skin lotion and nine controls. Urines were analyzed for total mercury by using cold vapor atomic absorption spectrophotometry. The mercury excreted for 6 hr before and 6 hr after DMPS treatment was 113 micrograms +/- 26 and 5037 micrograms +/- 682 S.E.M. for the skin lotion makers; 16.2 micrograms +/- 3.4 and 1410 micrograms +/- 346 S.E.M. for the skin lotions users; and 0.49 micrograms +/- 0.11 and 18.4 micrograms +/- 7.1 S.E.M. for the controls, respectively. The increases in urinary mercury resulting from the DMPS challenge test were 45-, 87- and 38-fold, respectively. The results demonstrate that, in humans exposed to mercurous chloride, DMPS increases the urinary excretion of mercury and that the DMPS/mercury challenge test is of value for a more realistic estimation of mobilizable mercury. An attempt to associate genotoxicity, as indicated by micronuclei content in buccal cells, with mercury exposure was inconclusive, perhaps because of the small number of subjects.