Phenotypic and genotypic characteristics of the peripheral blood mononuclear cells in nine patients with Felty's syndrome have been examined. One patient had an increased number and percentage of peripheral blood mononuclear cells with the phenotype CD3+ Leu-7+ CD16+ and showed a clonal rearrangement of the T cell receptor B chain gene. The remaining eight patients all showed a germline configuration of the T cell receptor B chain gene. In two patients an increased proportion of CD3+ Leu-7+ CD16- peripheral blood mononuclear cells (45 (SD 11)% of peripheral blood mononuclear cells) were found, while the remaining six patients had proportions of CD3+ Leu-7+ cells similar to those of patients with uncomplicated rheumatoid arthritis. These data confirm that patients with Felty's syndrome are heterogeneous, with at least three different peripheral blood mononuclear cell phenotypic subsets. One subset is characterised by a clonal expansion of an unusual lymphocyte subpopulation, another by polyclonal expansion, and the third subset has the same proportions of peripheral blood mononuclear cells as patients with uncomplicated rheumatoid arthritis.