A methodological study of hepatitis B surface antigen antibody (anti-HBs) was performed. Hollinger''s formula seems to provide a good accuracy when the enzyme immunoassay is employed. Moderate variations in the commercial anti-HBs kits studied were verified. Considering the results and the progress of the calibration curve of the WHO Reference International Standard, a calibration curve of the standard serum when high precision is required should be included.