SEVERAL reports have been published concerning the immunoglobulins of sublethally and lethally irradiated mice1–4. These have relied on the techniques of electrophoresis and immunoelectrophoresis. The former, although it can yield some quantitative information, does not adequately distinguish between different classes of immunoglobulin, and the latter, while offering excellent resolution of immunoglobulin classes, is only in the roughest way quantitative. These studies have indicated that a profound drop occurs in the concentration of gamma-globulins within a few days after lethal irradiation. The availability of unispecific antisera to each of four mouse immunoglobulins5—IgA, IgM, IgGl arid IgG2a (ref. 6)—and the development of the technique of single radial diffusion in agar whereby such antisera can be used to yield quantitative measurements of their corresponding antigen7 have now made it possible to obtain more precise information concerning the effects of radiation on the concentration of immunoglobulins.