Anti-CD44 MoAb IM7 induced the loss of CD44 from mouse leucocytes thereby inhibiting leucocyte migration and joint inflammation in murine arthritis. Thus, targeting CD44 with MoAb may have potential for the treatment of patients with inflammatory joint diseases. Expression of CD44 by peripheral blood (PB) and synovial fluid (SF) leucocytes from rheumatoid arthritis (RA) patients was compared and the ability of IM7 to modulate this expression determined. RASF lymphocytes showed increased CD44 expression compared with those in PB indicative of an activated phenotype. As inflammatory SF did not up-regulate CD44 expression on PB lymphocytes, the increased CD44 expression by SF lymphocytes was a result of the selective homing of CD44(high) cells to the synovium rather than an effect of the synovial environment. RASF granulocytes showed reduced CD44 expression compared with those in PB, again indicative of an activated phenotype. However, this reduction could be induced on PB granulocytes following culture with inflammatory SF and was inhibited by anti-TNF-alpha MoAb, implying that soluble factors in inflammatory SF such as TNF-alpha induced granulocyte activation and CD44 loss. IM7 induced the loss of CD44 from lymphocytes (both from PB and SF) and granulocytes in vitro, but was subsequently re-expressed after 24 h culture in the absence of the MoAb. This loss of CD44 was blocked by serine- and metalloprotease inhibitors implying that IM7 induced the proteolytic cleavage of CD44 by a mechanism similar to that reported for the loss of CD44 from PMA-activated granulocytes. Furthermore, IM7-treated CD44(low) lymphocytes showed reduced adherence to both an endothelial cell line and RA synovial fibroblasts in vitro. The unique ability of IM7 to reduce CD44 expression by lymphocytes suggests that it could prevent lymphocyte extravasation and synovial infiltration in RA as previously reported in murine arthritis.