The CD44 adhesion receptor is a hyaluronan-binding cell surface glycoprotein whose major isoform is a 95 kDa molecule with a wide tissue distribution. We have characterized two MAbs, W4/86 and RPN3/24, by immunochemistry, flow cytometry and fluorescence staining and demonstrated that they recognize the rabbit CD44 molecule. The results show that CD44 in the rabbit has a similar molecular size and cell distribution to human CD44. These antibodies have demonstrated for the first time, the presence of CD44 on synoviocytes. Preincubation of the rabbit synovial cell line HIG-82 with W4/86 inhibited its synthesis of PGE2 in response to autocrine synovial cytokines. These results indicate that the CD44 molecule may help to modulate synoviocyte metabolism.