Sera from animals, mainly of the Red Danish Milkbreed, have been examined by immunochemical analyses in order to select possible IgG‐2 immunoglobulin deficiencies. According to previous findings a low frequency was found in the healthy animal material (1 out of 417 animals) whereas the frequency of this immunoglobulin deficiency was strikingly much higher among diseased animals, especially among animals with pyogenic infections (13 out of 93 animals). It was assumed that lack of IgG‐2 may be associated with a reduced resistance to infections, in particular those caused by pyogenic bacte