The specificities of seven monoclonal antibodies to the human B cell differentiation marker CD24 have been investigated with respect to sialic acid containing carbohydrates. These are antibodies HB8, HB9, VIB-C5, VIB-E3, AL1a, LC66 and IB5, which are known to bind to polydisperse sialoglycoprotein(s) on Nalm-6 B lymphoblastoid cells. Three of the antibodies, HB9, VIB-E3 and IB5, have been found to resemble the first described antibody in this series, BA-1, in binding also to bovine submaxillary mucin. As with BA-1, the binding of the antibodies is abolished or reduced markedly after desialylation of the epithelial glycoprotein, and the binding to neuraminidase-treated Nalm-6 cells is also reduced. There is evidence for the involvement of of non-O-acetylated sialic acid in the determinants recognized by these antibodies, since there is a substantially enhanced binding following mild-alkali treatment of the epithelial mucin which removes O-acetyl groups. One of the antibodies, VIB-E3, is deduced to recognize the oligosaccharide sequences NeuAc alpha 2-6GalNAc and NeuAc alpha 2-6Gal as part of larger antigenic structures. This conclusion has been reached from the results of inhibition-of-binding experiments using a series of structurally defined sialo-oligosaccharides and direct binding experiments using oligosaccharides chemically linked to lipid (neoglycolipids).