Formalin has been recommended as an innocuous fixative for immunohistochemistry. However, several studies demonstrated impairment or blocking of antigenic activity of certain proteins. Formalin fixation was discovered accidentally by F. Blum in 1893 and its deleterious effects on various tissue structures were discussed extensively during the following decades. More recently, some authors assumed that formaldehyde bound to tissues can be largely or completely removed by washing and dehydration. According to chemical data, formaldehyde forms highly reactive methylols with uncharged amino groups. Such methylol groups yield methylene bridges with suitably spaced amides, arginine and aromatic amino acid sidechains. Only loosely bound formaldehyde is removed by washing for several hours. Residual bound formaldehyde cannot be dislodged by washing for weeks, but some formaldehyde is gradually removed when tissues are stored in water for an extended number of years. Methylene crosslinks resist treatment with high concentrations of urea, and can be broken only by drastic hydrolysis. It appears unlikely that such firmly bound formaldehyde is removed by conventional washing and dehydration procedures used in histochemistry. The superiority of methacarn, alcohol or acetone over formaldehyde fixation for immunohistochemical demonstration of prekeratin, myosin, type I and type IV collagen, laminin and fibronectin can be ascribed to the irreversible alterations of tissue proteins by formaldehyde.