The syndecan family of heparan sulfate proteoglycans (HSPGs) bind cells to extracellular matrix molecules and serve as receptors for growth factors. Using myeloma cells as a model, we have recently discovered that syndecan-1, via its heparan sulfate chains, also functions as a cell-cell adhesion molecule. (J. Biol. Chem. 270:5077). To directly address if this adhesion is specific for syndecan-1, myeloma cells were transfected with the cDNA for two other transmembrane proteoglycans: Syndecan-4 which shares a high degree of sequence homology with syndecan-1 and betaglycan which is structurally unrelated to the syndecans. Using rotation-mediated aggregation assays, we find that control-transfected cells remain predominately as a single cell suspension while cells expressing either syndecans-1, -4 or high levels of betaglycan form multicellular aggregates. Furthermore, cells bearing a mutant form of syndecan-1 lacking its normal cytoplasmic domain aggregate. This indicates that syndecan-mediated cell-cell adhesion is probably not dependent on an interaction with the actin cytoskeleton. Overall, these data indicate a new role for heparansulfate-bearing proteoglycans in mediating B lymphoid cell adhesion.