The results of photo-CIDNP measurements of Bence Jones proteins, immunoglobulins, and their proteolytic fragments are reported. The CIDNP spectra of constant region CL fragments, which were derived from lambda-type Bence Jones proteins with three different isotypes, gave polarizations originating from two Tyr and one Trp residues. From comparisons of the results obtained by using lambda-type Bence Jones proteins that possess the known amino acid sequences with those for the CL fragments, it was concluded that the CIDNP signals observed in the case of Bence Jones proteins are all due to the hypervariable region. On the basis of the results obtained by using IgG1, IgG2, IgG3, and IgG4 myeloma proteins along with their Fab, Fc, and pFc' fragments, it was concluded that CIDNP signals for the Fab region all originate from the variable region. The Fc region gives two Tyr polarizations, one of which has been assigned to Tyr-296. We have also shown that the second Tyr signal is due to Tyr-373. Interaction of Fc and staphylococcal protein A has also been examined by the CIDNP technique.