The aim of this study was to determine antibodies to Tamm-Horsfall protein subunits in patients with acute pyelonephritis. The protein subunits used in this determination were prepared by chemical treatment of Tamm-Horsfall protein isolated from the urine of healthy individuals. Values for IgG and IgA were significantly higher (p < 0.05 and p < 0.01 respectively) in patients than in healthy persons, while IgM class antibodies were significantly higher only in the test performed with subunits obtained with 8.3 mol/l acetic acid (THP-A) (p < 0.05). Values for all three classes determined in the test with THP-A were significantly higher in patients with vesicoureteral reflux than in patients with normal radiological findings (p < 0.05). Antibodies to Tamm-Horsfall protein subunits isolated from the urine of patients with acute pyelonephritis should also be determined.