Circulating T cells bearing receptors for the Fcportion of IgG (Tγ) were identified by sensitive immunofluorescent techniques with rabbit IgG b4 allotype/anti-b4 complexes. A twofold decrease in both proportion and absolute number of Tγ cells was found in patients with active systemic lupus erythematosus (SLE) relative to values obtained during disease remission. The reduction in Tγ cells was most evident in patients with severe hypocomplementemia. A deficit of Tγ cells in active patients was not demonstrated. The percentage of total T cells rosetting with sheep erythrocytes was reduced in peripheral blood of most patients regardless of disease activity status, but particularly during SLE exacerbation. Cells lacking intrinsic surface immunoglobulin, IgG Fc receptors, and receptors for sheep erythrocytes were increased. These cells, operationally termed null. exhibited an inverse linear relationship with T cells that was not apparent in regression analyses performed against other lymphocyte subpopulations. Such differences were not found for B cells and IgG receptor-bearing non-B/non-T cells which were present in normal proportions in virtually all patients. The origin and functional significance of these unusual lymphocyte subpopulation abnormalities are discussed.