Mercury (Hg)-, selenium (Se)-, and arsenic (As)- containing proteins in liver tissues of bighead carp and grass carp sampled from a mercury-polluted area of Wanshan, Guizhou Province, China, were separated by thin-layer isoelectric focusing. The relative content of Hg, Se, and As in protein bands was measured with synchrotron radiation x-ray fluorescence. The results indicated that there were at least 3 Hg-containing bands with pI 3.7, 4.8, and 6.2 in liver of bighead carp and 1 Hg-containing band with pI 6.2 in grass carp. Se and As were found in the Hg-containing bands 3.7 in bighead carp and 6.2 in grass carp. The bands may be corresponding to the antagonistic effect of Se against the toxicity of Hg and As. In addition, Hg and As often coexist in the same band, suggesting that the two elements may be involved in the same metabolic processes.