Human spleen cells were investigated for specific oestrogen binding using oestradiol-17β. High affinity, low capacity binding was observed (Kd=2.5±10−10m) and the characteristics of the oestrogen receptor identified were typical of this type of species. T lymphocyte subpopulations were prepared by negative selection by means of the monoclonal antibodies Leu 2a and 3a. Oestrogen receptors were only found in T cells of the suppressor/cytotoxic (CD8-positive) subset while helper/inducer lymphocytes (CD4-positive) showed no significant steroid binding by cytosol and nuclear assays. These results are supported by data from experiments into the effects of oestradiol-17β on immunoglobulin secretion, where it was apparent that only the suppressor/cytotoxic T-cell subpopulation was responsive to the presence of the steroid.