Mutants of hepatitis B virus surface antigen (HBsAg) are known as a cause of false-negative results in diagnostic tests for HBsAg; particularly when a diagnostic kit utilizes monoclonal antibodies to detect HBsAg. We compared seven HBsAg kits with regard to sensitivity for HBsAg subtypes (ad, ay) and their ability to detect nine different HBsAg mutants. Among them, the sensitivities of five kits were high and comparable to each other (0.2 - 0.3ng/ml). However, two kits were of lower sensitivity (0.8-1.3ng/ml, and 2.4-2.5ng/ml, respectively). Two kits, produced by the same company, reacted with all of the nine HBsAg mutants, but five kits showed false-negative results with one or more of the HBsAg mutants. These data indicate that there are differences in the detection sensitivities for HBsAg and abilities to detect HBsAg mutants among commercially available HBsAg kits, which may explain false-negative clinical results.