One-third of patients with Felty's syndrome (FS) have significant clonal expansions of CD3+ CD8+ large granular lymphocytes (LGLs) in their peripheral blood. The reasons for this are unclear, but one hypothesis is that they are activated antigen-specific cells of pathogenic relevance. Cytofluorographic analysis of activation markers demonstrated that the cell surface phenotype of these expansions was CD57+, HLA-DR+, IL-2R-, Leu-8+, CD69+, LFA-1+, ICAM-1+, VLA-4+, i.e. 'activated' T cells. However, they also expressed the phenotype CD45RA+, CD45RBbright, CD45RO-, usually associated with 'naive' cells. This could result from aberrant activation, malignant transformation or from a 'reversal' of CD45 phenotype following chronic antigenic stimulation. In three patients with RA and non-clonal LGL expansions, a more variable phenotype was found. In one of these patients, the expanded population was identified in the peripheral blood, but not the synovial fluid. This may suggest that, at least in this individual, any pathogenic effect is exerted systemically.