HLA-DP+T cells in peripheral blood from 23 patients with systemic lupus erythematosus (SLE) were examined using two-colour flow cytometry analysis. A marked increase of HLA-DP+T cells was observed in patients with SLE (20.5–98.7%; 59.8±20.8%) in comparison to normal subjects (1.3–20.6%; 11.1±7.2%), and the ratio of these cells greatly exceeded that of the HLA-DR+ T cells (6.5–49.1%; 21.5±12.7%). This high frequency of HLA-DP+ T cells in patients with active SLE decreased with predonisolone therapy. When the lymphocytes from normal subjects were stimulated with PHAin vitro, HLA-DP+T cells increased from 1.8 to 59.2%. Therefore, it appears that the HLA-DP antigen expression on T cells is a practical marker for monitoring changes in the proportion of activated T cells in patients with SLE during the course of therapy.