The density of the three principal immunoglobulin containing cells (IgA, IgM, IgG) in the human jejunal mucosa was determined in eight adult celiac patients and in eight subjects without any gastrointestinal abnormalities, using immunofluorescent techniques. A statistical comparison of the results showed a particularly significant increase of the IgM cells and a less significant increase of the IgG cells in celiac disease. A decrease of the IgA cells was also found in celiac disease but with no statistical significance. These results are discussed in relation to other phenomena observed in the intestinal wall of celiac patients and indicate the importance of the immunoglobulins in this disease. One particular celiac patient with a high IgG cell count who later developed lymphosarcoma of the intestine is discussed in detail.