B cell immunoglobulin production is regulated by helper T cells through direct interaction and secreted cytokines. In the present study, we functionally analyzed CD27 in cord and peripheral blood B cells. Adult peripheral blood B cells were separated into CD27⁺ and CD27⁻ cells, which differed in their morphology. Cord blood B cells did not express CD27, and CD27 expression on peripheral blood B cells increased with age. Only CD27⁺ B cells had the ability to produce immunoglobulin, which was increased by contact with a tumor necrosis factor‐related transmembrane ligand, CD70. Adult peripheral blood CD27⁺ B cells can be further subdivided into two discrete subtypes: IgD⁻CD27⁺ and IgD⁺ CD27⁺ B cells. IgD⁻ CD27⁺ B cells produce IgG, IgM and IgA, whereas IgD⁺ CD27⁺ B cells predominantly produce IgM. The addition of activated CD4⁺ CD45RO T cells expressing CD70 caused down‐regulation of CD27 expression on activated B cells, and this down‐modulation was completely blocked by anti‐CD70 monoclonal antibody, indicating direct T‐B cell contact via CD27/CD70. The triggering via CD27 and CD40 additively increased the immunoglobulin production under Staphylococcus aureus Cowan strain plus interleukin‐2 stimulation. Taken together, our findings demonstrate that peripheral blood B cells are separated into subpopulations by CD27 and IgD expression and that CD27⁺ B cells produce large amounts of immunoglobulin by interaction with the CD70 molecule.