The systemic amyloidoses are a heterogeneous group of congophilic fibrillar protein deposition diseases that should be subtyped chemically by immunohistologic methods. Biochemical methods sometimes are required to confirm or identify the amyloid type in unfixed or informalin-fixed tissue samples. We report the results of formic acid extraction and immunochemical and biochemical characterization of deposits informalin-fixed tissue samples from 10 cases of amyloidosis and 3 from nonamyloid monoclonal immunoglobulin light chain deposition disease. The results in 11 of 13 cases demonstrated concordance with the previous immunohistochemical and/or biochemical data obtained in unfixed tissue samples from the same specimens, and in 2 of 13, the protein deposits that previously could not be classified by standard immunohistochemical methods were identified by amino acid sequence. An additional new finding of constant-region rather than variable-region fragments as the major constituent protein in 1 case of lambda light chain amyloidosis demonstrated the value of the method and its importance for future applications.