Male mice received a single subcutaneous injection of 10 mg/kg body weight of Ag as AgNO3 and were killed at 2, 6, 10 and 24 h after the injection. Serum ceruloplasmin (Cp) significantly decreased at all times. These low Cp values were accompanied by low serum Cu levels. There was a significant (P less than 0.01) positive correlation between the two substances. A significant enhancement of hepatic Cu concentration was due to the appearance of low Mr protein induced by the Ag administration. The LMW protein induced by Ag was associated with three metals (Ag, Zn and Cu), the Mr values of which ranged from 8300-10 300. In the hepatic supernatant fraction, Ag was recovered at 0.9, 2.7, 9.4 and 8.7% of the doses at 2, 6, 10 and 24 h, respectively. Unexpectedly, the proportion of Ag existing in the low-Mr protein was only 65% of total supernatant Ag, even at 24 h. The decrease in Cp activity with a concomitant increase in hepatic Cu suggests that the increased hepatic Cu was not utilized for the addition to apo-Cp.