Recombinant soluble (s) CD25 inhibit T-cell proliferation. (a) Recombinant sCD25 was added to OKT-3/interleukin-2 (IL-2) stimulated peripheral blood mononumear cells in triplicates from 11 donors in an Alamar Blue assay. Cells were cultured in final concentrations of 1000 (n = 8), 500 (n = 8) 250 (n = 8) or 100 pg/ml (n = 3) sCD25. The experiment was repeated four times with similar results. (b) PBMCs from six healthy donors were unstimulated, or stimulated with anti-CD3 (OKT3), OKT3 + IL-2, OKT3 + IL-2 + sCD25 (1000 pg/ml) or OKT3 + pre-incubated IL-2/sCD25 to allow complex formation before culture. The OKT3/IL-2 T cells were significantly different from unstimulated (P = 0·002), OKT3-stimulated (P = 0·02), OKT3/IL-2/sCD25-stimulated (P = 0·0152) and OKT3 + IL-2/sCD25 complex (P = 0·02). The experiment was repeated with six additional donors with similar results. Mann–Whitney U-test was used to analyse differences among groups.