Human-murine myeloid somatic cell hybrids were assayed for the expression of the myeloid-associated sialyl-X determinant. This determinant is expressed at the surface of hybrid cells containing human chromosome 11, but its expression could not be correlated with the presence of the sialyltransferase which is involved in the sialyl-X synthesis. The sialyl-X determinant, however, is simultaneously expressed with another alpha 2----3-sialyltransferase activity, which is involved in the sialylation of the O-linked Gal beta 1----3GalNAc alpha-R core structure. Chromosomal analyses and enzymatic data suggest that human chromosome 11 is involved in the expression of both the sialyl-X antigen and this alpha 2----3-sialyltransferase.