Circulating antibody to Tamm-Horstall protein (THP) was measured using a radioimmunoassay in forty-five patients on maintenance hemodialysis and compared to levels of antibody titers measured in sera from ten healthy controls. The etiology of the end-stage kidney disease in the patient population was polycystic kidney disease in thirteen, glomerulonephritis in fourteen, diabetic nephropathy in nine, interstial nephritis and chronic pyelonephritis in three each, multiple myeloma in two, and urinary tract obstruction in one. Four patients had significantly elevated titers of antibody to THP but shared no other unifying characteristics. The results also indicate that none of the groups studied had mean antibody titers significantly different from controls. Furthermore, no general trend was apparent between levels of antibody to THP and number of months on dialysis. Observations made during the study revealed that heparinized samples of blood had lower titers of antibody to THP than did non-heparinized samples from the same patient. This finding was repeated when other anti-coagulants, i.e., ethylenediaminetetraacetate (EDTA) and sodium citrate, were used. Titers returned toward normal when CaCl2 was added back to samples anticoagulated with EDTA and sodium citrate. This suggests that clotting factors, probably fibrinogen, interfered with the measurement of antibody titers. Therefore, only serum should be used in further investigations of THP antibody using this assay.