Regulatory T cell subpopulation (Treg) are isolated for a human host by selection for cells expressing CD4 and CD25. The Treg cells are further characterized by expression of CTLA-4, CCR6, and CD30. In addition, the Treg cells are CD62Lhi, CD45RBlo, CD45ROhi, CD45RA−. The Treg cells of the invention reflect the immunologic status of the donor, in terms of the number, location and T cell antigen receptor specificity of the Treg cells. This information is used in diagnostic assays relating to immunologic disorders, e.g. cancer related immunosuppression; autoimmune disorders; atopic states, etc. The isolated Treg cells are useful in transplantation, for experimental evaluation, and as a source of subset and cell specific products, including mRNA species useful in identifying genes specifically expressed in these cells, and as targets for the discovery of factors or molecules that can affect them. Culture assays and systems of interest include the interactions of Treg cells with immature and mature dendritic cells, interactions with T cell subsets, responsiveness to antigen specific and non-specific stimulus, and the like.