Over-expression of P-glycoprotein (P-gp) in lymphocytes is implicated in the failure of immunosuppressant therapy. We investigated P-gp function in peripheral-blood CD3+, CD4+, and CD8+ cells of 14 healthy subjects and 12 patients with systemic lupus erythematosus (SLE). P-gp function was estimated by the transporter activity of the cells based on the efflux of Rhodamine-123 (Rh123) from the cells in the presence or absence of a P-gp inhibitor, cyclosporine A. P-gp function in the CD8+ cells of the healthy subjects was significantly higher than that of the SLE patients (p=0.0318), whereas the function in CD3+ cells and CD4+ cells were not significantly different between the healthy subjects and the SLE patients. The patients were divided into two subgroups according to their clinical response to glucocorticoid (GC) therapy, i.e., a high-response group (HR) (n=6) and a low-response group (LR) (n=6). In contrast, P-gp function in CD4+ cells of the LR group was significantly higher than that of the HR group (p=0.0432). Further, no significant differences in the P-gp function in CD3+ and CD8+ cells were observed between the two groups. The data showed a relationship between clinical sensitivity to GC therapy and P-gp function of CD4+ cells in SLE patients. Thus, the estimation of P-gp function in peripheral-blood CD4(+) cells might be useful for the estimation of clinical response to GC therapy.