The presence of the antiparallel-&pleated sheet conformation in isolated human amyloid protein fibrils has been confirmed by infrared spectroscopy. In most amyloid samples, this conformation was enhanced by acidic solution conditions. Infrared spectroscopy (Amide I and Amide V absorption bands) and x-ray diffraction methods were also used to examine the immunoglobulin molecule for solid state @-structure. It was found that both heavy chains and Bence Jones proteins exhibited some @-pleated sheet content upon acid and/or heat treatment. Furthermore, pepsin digests comprising either the variablerich region (Fd') of the immunoglobulin heavy chain or, in particular, filamentous variable segments of x and A Bence Jones proteins were, as isolated, very similar to amyloid in @-structure content. Data from other immunoglobulinderived samples did not exhibit extensive @-pleated sheet content. On the other hand, most smyloid and immunoglobulinderived samples did display some @-structure when cast from 50% HCOOH solution. Under these conditions, however, filamentous light chainvariable segments exhibited welldefined infrared patterns rich in antiparallel+-pleated sheet structure and gave a "cross-@" x-ray diffraction pattern.