Following recent observations using monoclonal antibodies that carbohydrate structures behave as differentiation antigens of man and mouse, we have made a preliminary survey of the expression of 8 monoclonal antibody-defined carbohydrate antigens on blood cell smears of man, baboon, mouse, rat, rabbit, pig, and dog. There are considerable species differences in the patterns of antigen expression. However, certain generalizations can be made as follows: the i and I antigens, associated with linear and branched carbohydrate chains consisting of repeating N-acetyl-lactosamine sequences (Gal beta 1-4GlcNAc, termed Type-2 backbone sequences) are widely distributed among granulocytes and lymphocytes of all the species studied, and on erythrocytes, monocytes, and platelets of some of them. Substantial amounts of Type-1 backbone sequences (Gal beta 1-3GlcNAc) may occur on rabbit lymphocytes. The N-acetylneuraminic acid-containing antigens, Pr2 and Gd, are also expressed to varying degrees on blood cells. On the other hand, antigens based on mono- and difucosylated N-acetyllactosamine, termed SSEA-1 (or X-hapten) and C14 (or Y-hapten) are predominantly granulocyte/monocyte-associated antigens. The former antigen is expressed in overt form only on untreated human granulocytes but occurs in cryptic state, masked by sialic acid, on human monocytes, and on the granulocytes and monocytes of baboon, rabbit, and dog but not on those of mouse, rat, and pig. The latter antigen is expressed on human granulocytes and on neuraminidase-treated monocytes and granulocytes of dog. Lymphocytes of dog are unusual in their expression of C14 antigen, in cryptic state, masked by sialic acid residues. Although the physiological roles of these various carbohydrate structures, in vivo, are not yet known, they seem excellent candidates as determinants of species and cell-type differences in susceptibilities to infective agents.