Serum levels of alpha 1-antitrypsin (alpha 1-AT) and alpha 2-macroglobulin (alpha 2-M) and, as controls, alpha 1-acid glycoprotein (alpha 1-AG) and haptoglobin were evaluated by means of laser nephelometry in 17 patients with acute viral hepatitis (AVH) type A, 16 with AVH-B, 12 with AVH-NANB and 8 with fulminant hepatitis B. On admission, alpha 1-AT levels were elevated in one third of AVH-A and AVH-B cases, but subsequently declined; alpha 2-M levels were elevated in about 40% of AVH-B patients during the 2nd, 3rd and 4th week after admission. No significant correlation was found between elevated levels of protease inhibitors and aminotransferase values or drug addiction and delta coinfection. alpha 1-acid glycoprotein and haptoglobin levels were always normal or low. Protease inhibitors did not show any elevation in fulminant hepatitis, while changes were found only in a few patients with AVH-NANB. Thus, no clearcut pattern of changes in protease inhibitors has been found in association with each type of hepatitis, although alpha 1-AT and alpha 2-M elevations are mainly found in AVH-B.