The two-site enzyme-linked immunosorbent assay (ELISA) for lactoferrin using polyclonal antibodies to spatially distant epitopes has been developed. The assay sensitivity defined as minimal detectable lactoferrin concentration for p = 0.05 is 0.5 ng/ml. Accuracy of the assay (variance coefficient) is 7% within the clinical range of antigen concentrations. Human albumin, hemoglobin, and transferrin in concentrations up to 5 mg/ml practically do not interfere with the measurement. Sera of healthy donors and viral hepatitis patients were investigated using the two-site ELISA. The lactoferrin content in 44 donors' sera was 130 +/- 40 ng/ml (medium +/- standard deviation). A study of the serum specimens of 95 patients with hepatitis A, B, and C showed significant increase in serum lactoferrin concentration: 850 +/- 420, 780 +/- 580, and 680 +/- 500 ng/ml respectively. The assay showed good characteristics and may be recommended for lactoferrin measurement in patients' sera.