The levels of serum IgD and of the circulating immune complexes (CIC) were determined in 168 diabetic patients, of whom 78 with type 1, 59 with type 2 and 31 with the so-called "intermediary" type of the disease, in comparison with 124 non-diabetic subjects for IgD and 100 for CIC. The results revealed very low IgD titres (less than 1 mg%, considered undetectable) in almost 3/4 of the cases; values over 1 mg% were recorded mostly in the cases of type 1, followed by those of "intermediary" and of type 2 diabetes. The mean CIC values of 67.13 +/- 36.53 optic density units (O.D.U.) were significantly higher than in the non-diabetic controls. Certain differences with respect to age, diabetes type, duration of the disease and of the insulin therapy were also recorded. The data are interpreted with caution, further investigations being necessary to the assertion of definite conclusions.