Immunodetection techniques are dependent on enzyme–protein conjugates for the visualization of antigen–antibody complexes. One of the most widely used is the avidin–biotin–peroxidase complex (ABC) method. However, treatment of certain tissues with ABC reagents alone may result in high background, which is indicative for the presence of endogenous biotin or biotinylated proteins. In goldfish and salamander retinal sections, we observed a distinct staining pattern, presumably through binding of avidin to endogenous biotin in M�ller cells. These findings summon for caution in the application of detection systems based on biotinylated antibodies or biotinylated DNA probes.