The Ia (I region associated) antigenic system in the mouse is coded for by genes located within the H-2 complex and consists of at least four subregions (IA, IB, IE, AND IC) which code for a minimum of 21 (Ia. 1 to 21) specificities (1-3), although the existence of IB and IE subregions is now being questioned. These antigens were first detected on the surface of B lymphocytes and were subsequently found on macrophages, epidermal cells, and sperm cells (4). It is also apparent that some Ia specificities are present on either resting and activated T-blast cells (2-8). In addition, we have detected considerable amounts of Ia antigenic material in mouse serum and other tissue fluids, particularly after antigenic stimulation (8, 9). The current interest in this system stems from the close involvement of the I region in immune responses (1-3), especially with the recent demonstration that soluble factors of both antigen-specific and nonspecific nature which are involved in T-B cell collaboration (10, 11) and suppression (12), also carry Ia antigens. In this context, we have found that although B cells have a higher density of Ia antigens on their surface than T cells, the Ia antigens in serum are derived from T cells (9).Biochemical studies of the Ia specificities have proceeded along two lines. The cell surface Ia antigens of B cells have been extracted with detergents and were found to be glycoproteins with mol wt of approximately 30,000 (13,14). By contrast, our chemical studies of the serum Ia material suggest that it is predominantly (but not exclusively) carbohydrate in nature as the material has the following properties: (a) It is rich in carbohydrate and almost devoid of protein (15); (b) The Ia antigenicity is destroyed by neuraminidase and periodate, but not by pronase (15); (c) The Ia antigen binds to concanavalin A and Lotus lectins (15); (d) It is extracted and purified from serum by methods identical to those used to prepare oligosaccharides from human milk (15-17); (e) Its behaviour on paper and thin layer chromatography is similar to that of other oligosaccharides (15), and after acid hydrolysis of the purified Ia product a number of simple sugars have been detected by gas-liquid chromatography (manuscript in preparation). In this paper, we describe hapten inhibition studies using a variety of sugars that show that for at least four Ia antigens (Ia.1, Ia.3, Ia.7, and Ia.15) the antigenic specificity is defined by sugars.