In a group of 84 patients with exacerbated chronic pyelonephritis the authors have determined the blood serum levels of ceruloplasmin, haptoglobin and sialic acid before, during and after antibacterial treatment. They have found a decrease in these levels following treatment. The results of ceruloplasmin, haptoglobin and sialic acid assays could be used as an index for the activity of the pathologic process, as well as in the differential diagnosis between pyelonephritis and glomerulonephritis.