The inactivation with deuterons of bovine serum albumin molecules adsorbed to a plane surface of barium stearate is found to be identical with that of albumin monolayers made by spreading the protein on a water surface and transferring to a metal plate. The activity of the BSA is measured by its ability to combine specifically with the homologous rabbit antiserum. Better experimental conditions in this work made it possible to distinguish two types of antigenic sites operating in BSA molecules, a large type involved in about 75% of the serological activity, and a much smaller site. The larger type is roughly spherical with a molecular weight of about 7000 or 10 % of the molecule, but the smaller site is probably less than 1 % of the size of the molecule.