A new oral agent, N-acetyl-DL-penicillamine, effective in protecting rats against the lethaleffects of mercuric chloride, is described. This compound is more effective than either D-penicillamine or DL-penicillamine in this respect. Data are presented on the toxicity in the rat ofthese orally effective sulfhydryl compounds, indicating that N-acetyl-DL-penicillamine and D-penicilamine are less toxic than DL-penicillamine.