The effects of cadmium on the peak area of metallothionein (MT) protein were studied by capillary zone electrophoresis with a polyacrylamide-coated tube at neutral pH. When cadmium was added to a commercial standard MT-1 isoform prepared from rabbit liver, the peak are of the MT-1 isoform decreased in a time- and dose-dependent manner. The MT-2 isoform also decreased with time, but the rate of decrease was lower than that of the MT-1 isoform. When ethylene glyco-bis-(beta-aminoethyl ether)-N,N,N'N'-tetraacetic acid (EGTA) was added to a solution containing cadmium and MT-1, the peak area recovered with increasing concentration of EGTA. Zinc caused a slight decrease in the peak areas of MT-1 and MT-2 compared with cadmium, while addition of sodium did not decrease the areas. Furthermore, the peak areas of MT isoforms of the crude extract prepared from zinc-treated mice also decreased with increasing concentration of cadmium. These results indicate that cadmium may changed the charge of MT, which may account for the observed differences in electrophoretic behavior.