Reactive arthritis (ReA) and rheumatoid arthritis (RA) are inflammatory joint diseases which differ in several clinical and immunological aspects. In both diseases locally activated T cells are considered important for pathogenesis. Subsets of T cells are demonstrated by staining with monoclonal antibodies recognizing isotypes of the common leukocyte antigen: CD45RA, which is considered to be a marker of native T cells, and CD45RO a marker of primed T cells; a third subset was also identified according to the staining intensity of CD45RB: CD45RBbright and CD45RBdim. The expression of these and the surface markers CD3, CD4, CD8, CD14 and CD56 was studied in synovial fluid (SF) and peripheral blood (PB) of patients with ReA (n = 11), RA (n = 10) and normal controls (n = 10) by three-colour immunocytometry. Significantly more CD45RBbright were found in the PB of patients with ReA (64.1%) and RA (63.5%) than in the SF of ReA (36.8%) and RA (40.6%) patients. However, when analyzed in relation to CD45RO, the CD45RO/CD45RBbright population was not significantly different between the SF and PB of patients with ReA (32.2% vs. 26.8%) and RA (30.6% vs. 28.1%), respectively. Among the three cellular subtypes of CD4+ lymphocytes demonstrated in this study (CD45RA/RBbright, CD45RO/CD45RBbright and CD45RO/CD45RBdim) none, in contrast to previous results of other groups, showed marked differences between ReA and RA in either of the compartments examined.