OBJECTIVE Human mucosyl lymphocyte marker (HML-1) antigen is an activation antigen and adhesion molecule of the beta 7 integrin family, which is generally restricted to T cells found in the intestinal epithelium. Expression of the membrane antigen as defined by the monoclonal antibody HML-1 was studied on peripheral blood (PB) lymphocytes and synovial fluid (SF) lymphocytes in 10 patients with rheumatoid arthritis (RA) and in a control group of patients with osteoarthritis (OA). METHODS Double fluorescence activated flow cytometry was used to assess HML-1 expression with T cell subtype antigens (CD3, CD4, CD8) or activation markers interleukin 2 receptor (IL-2R) (CD25), HLA-DR, and lymphocyte function associated antigen (LFA-1) (CD11a) were assessed by flow cytometry. RESULTS HML-1 antigen expression in PB lymphocytes of patients with RA (7.3%) was found to be comparable to the control group (6.4%). In contrast, 25.4% (range 14-43%) of SF lymphocytes expressed HML-1 antigen, compared to 13.6% of SF lymphocytes in patients with OA (p < 0.001). In RA, 62% of HML-1 positive cells from SF lymphocytes were the CD8 subtype, compared to 10.6% of PB lymphocytes (p < 0.003), and 18% of control SF lymphocytes (p < 0.05). Furthermore, HML-1 antigen and HLA-DR antigen were coexpressed in 75% of RA SF lymphocytes compared to 29.6% of control SF lymphocytes (p < 0.01). In contrast, coexpression of LFA-1 and the Il-2R did not differ from that of control. CONCLUSIONS We describe overexpression of the adhesion molecule HML-1 in SF lymphocytes of patients with RA, preferentially in the CD8 subset. These results suggest a similarity between the expression of activation antigens in SF lymphocytes of patients with RA and T lymphocytes present in the intestinal epithelium.