Hemophiliacs often have mild anemia, and hemolysis has been suggested as the likely mechanism on the basis of the reduced serum haptoglobin values frequently observed in these patients. It has been suggested that hypohaptoglobinemia results from isohemagglutinins or other contaminating proteins in the infused factor concentrates. The advent and increased utilization of Factor VIII concentrates that are highly purified by use of monoclonal antibodies have provided the opportunity to study whether proteins other than Factor VIII contained in the concentrate induce hemolysis. Of 49 consecutively studied Factor VIII-deficient hemophiliacs, 19 (39%) had a reduced serum haptoglobin level (less than 27 mg/dl). In particular, 16 of 35 (46%) of patients receiving only monoclonally purified Factor VIII products (Monoclate or Hemofil-M) had a reduced serum haptoglobin value. Haptoglobin measurements were variable on repeat measurement in 8 patients. Haptoglobin levels did not correlate with type or severity of hemophilia, hemoglobin value, or alterations in liver function. Low serum haptoglobin values were also observed in children with leukemia, without apparent hemolysis, who had extensive cutaneous hemorrhage associated with thrombocytopenia. We propose that reduced serum haptoglobin values in hemophiliacs do not result from immune-mediated hemolysis due to contaminating proteins in the concentrate. Moreover, hypohaptoglobinemia may not be due to hemolysis at all but may instead result from dissolution of hematomas and other foci of internal hemorrhage.