An important role for the CD27/CD70 ligand pair is emerging with respect to lymphocyte maturation in the periphery, while the putative role of CD27 and CD70 in thymic selection awaits further research. Triggering of the CD27/CD70 route affects levels of proliferation and differentiation of different lymphocyte subpopulalions upon antigenic stimulation in different ways, and accumulating data support the notion that the receptoriigand pair controls proliferation and facilitates maturation processes without interfering with programmed details of individual cells. It appears to control the crosstalk of the different subsets with each other both within the Т. В. and N'K celllineages as well as between them. Its emerging role in synergy with CD28 and CD40 routes overlaps the immune response with respect to the T helper-mediated cascade of T and В cells, but allows extension of this help to the NK cells as well. On the other hand itdoes not (as far as current data go) appear to affect the inflammatory response involving nonlymphoid cells, as reported for CD28 and CD40, or directly influence the interaction with antigen-presenting cells.