A study was made of the serum levels of several enzymatic activities in guinea pigs treated orally for 40-50 days with lead (120 mg/kg body weight). A marked increase in the serum levels of liver-specific enzymes such as sorbitol-dehydrogenase was observed, indicating the existence of a liver damage in this experimental condition. Both cytoplasmic (sorbitoldehydrogenase, lactatedehydrogenase and glutamic-oxalacetic-transaminase isoenzyme 1) and mitochondrial (glutamatedehydrogenase and glutamic-oxalacetic-transaminase isoenzyme 2) enzymes were increased in the serum, while normal levels of lysosomal enzymes such as acid ribonucléase and acid phosphatase were observed. An increase in the serum levels of enzymes of erythrocytic origin was also observed. The findings are discussed.