Nonspecific staining appears in immunohistological preparations in a multifactorial way and in variable degrees. Because the so-called “background” reactions often affect the interpretation of results, appropriate controls are necessary. Apart from natural antibodies, cross-reacting antibodies, Fc receptors etc. which can be the reason for non-specific immunostainings, various other factors are responsible for background reactions. In order to solve background problems at least to some extent, blocking solutions can be employed. For routine work, a variety of blocking solutions have been formulated which enable reduction of some major causes such as the reduction of hydrophobic binding, ionic and electrostatic interactions, blocking of endogenous enzyme activities or blocking of avidin and biotin interactions.