The changes resulting in proteins, and various protein derivatives, when mixed with formaldehyde solutions have interested many investigators. The problem has been approached from both the chemical and the immunological viewpoints. Although much has been added to our knowledge of these reactions, there still remain many unsettled questions. The exact site in the protein molecule to which formaldehyde attaches itself, the extent of the resulting change, and the immunological characteristics of such modified proteins are not yet completely clear. Benedicenti (1), Schiff (2), and Schwarz (3) found the aldehyde radicle to unite with the free amino groups of the protein molecule. Pick (4) confirmed these observations but believed also that significant structural alterations occurred in the protein molecule after the union with formaldehyde. Kossel and Gawrilow (5) held that only lysine containing proteins united with the aldehyde, and later Kossel and Edelbacher (6) included those which contained histidine.