Different adhesive capacity in interactions with CD55 has been ascribed to the isoforms of the leukocyte CD97 antigen, CD97 (EGF 1,2,5), CD97 (EGF 1,2,3,5), and CD97 (EGF 1,2,3,4,5). In the study, coexpression of the three CD97 isoforms and predominance of CD97 (EGF 1,2,5) transcripts in leukocytes are demonstrated. The contribution of CD97 (EGF 1,2,3,5) and CD97 (EGF 1,2,3,4,5) to total CD97 levels varied among most cell types only slightly, although relatively higher mRNA levels of both isoforms were detected in U 937 cells and monocytes. In peripheral blood lymphocytes, CD97 isoforms did not show clear variation after PMA stimulation and were down-regulated equally after CD97 cross-linking. Moreover, the CD97 isoform pattern was not altered in monocytes after interferon-gamma stimulation and in synovial T cells from patients with rheumatoid arthritis. CD97 mRNA levels did not necessarily correspond to CD97 surface density. The findings suggest that adhesive activity of CD97 toward CD55 is unlikely to be regulated by differential CD97 isoform expression.