Newly introduced BAS method in immunoperoxidase staining was compared with PAP method and the advantages and disadvantages of BAS method were discussed. Various dilutions of primary antisera of IgG, aFP, lysozyme, HSV II and GFA were applied to the surgical biopsy specimens of inflammatory reaction, hepatocellular carcinoma, gastric adenocarcinoma, cervical dysplasia and astrocytoma, respectively, and each specimen was stained with PAP or BAS method. The ratio of specific staining-background staining was higher in BAS method, where dilutions of the primary antisera were 4~16 times those applied in PAP method. The high dilution titers in BAS method was thought to be due to strong binding activity between biotin and avidin, and also due to lattice-like structure of ABC containing many peroxidase molecules. The main disadvantage of BAS method is endogenous avidin binding activity which can be overcome by pretreatment with avidin and biotin.