Similar to activated T cells, LB T cell lymphoma expresses the CD44 cell surface Ag. In addition, the vast majority of LB cells also express the beta 2 (CD18) and alpha L (CD11a) chains of LFA-1 integrin. In view of the finding that anti-CD18 mAb blocked spleen, but not lymph node invasion by LB cells inoculated s.c. into BALB/c mice, we tested the ability of anti-CD44 mAb (IM 7.8.1) to block the infiltration of LB cells into the lymph nodes. We found that, as opposed to anti-CD18 mAb, anti-CD44 mAb, as well as its F(ab')2 or Fab fragment, prevented lymph node infiltration but had no effect on spleen invasion. This conclusion was based on histologic examination and [3H]thymidine incorporation into proliferating LB cells invading the lymphoid organs. Histologic analysis further demonstrated that LB cells invade the lymph node via the afferent lymphatics. The surface expression of CD44 molecules on LB cells was enhanced after PMA activation. PMA activation also enabled in vitro binding of the lymphoma to hyaluronic acid (HA), a known ligand of CD44. Because anti-CD44 mAb, its F(ab')2 or Fab fragment, and hyaluronidase blocked this binding, we also tested the ability of the enzyme to inhibit lymph node invasion by LB cells. We established through histologic examination and [3H]thymidine incorporation that hyaluronidase protected the lymph node, but not the spleen, from invasion by the lymphoma.