Serial white cell and differential counts in 114 patients receiving 131 courses of cobalt-60 radiotherapy revealed the following findings: (1) Large field cobalt-60 radiation was associated with eosinophilia in 30% of all such courses. (2) Eosinophilia was most frequent in patients receiving pelvic radiation unrelated to the type of disease treated. (3) Typically the eosinophilia increased progressively to peak values in the 4th week of pelvic radiation. The possibility of antigen-antibody reactions explaining radiation-related eosinophilia (RRE) was investigated by serial observations of serum immunoglobulins.