CD44 molecules are comprised of multiple alternatively spliced forms and are associated with diverse functions such as mediation of carcinoma metastasis and T cell coactivation. To study the function of individual CD44 isoforms, we have transfected CD44 isoforms into CD44-negative Jurkat T cells and produced cloned Jurkat cell lines that are stably transfected with either a CD44 isoform containing no alternatively spliced insert (CD44H) or a CD44 variant (CD44E) containing an insert of 132 amino acids derived from exons 12, 13, and 14 of the CD44 gene. We found that neither CD44H- nor CD44E-transfected Jurkat T cells constitutively bound hyaluronan (HA), whereas PMA treatment induced Jurkat cells transfected with CD44H but not CD44E to bind HA. CD44 mAb against noninsert regions of the CD44 extracellular domain (A3D8, A1G3) and polyclonal antisera against the COOH-terminal extracellular glycosaminoglycan region of CD44H (anti-6A serum) both induced CD44H-transfected cells to bind HA, whereas only one CD44 mAb (A1G3) induced CD44E-transfected Jurkat T cells to bind HA. Studies of Jurkat cells transfected with CD44H forms with truncations of the CD44 cytoplasmic domain demonstrated that the cytoplasmic COOH-terminal 52 amino acids were critical for binding of HA to the CD44 extracellular domain. Thus, these data underscore the importance of the CD44 cytoplasmic domain in the function of the extracellular portion of CD44H, and demonstrate a role for ligation of human CD44 isoforms at multiple distinct sites in regulation of expression of CD44 binding to HA.