Like interleukin (IL)-2 and IL-4, IL-7 can act as a growth factor for activated T lymphocytes. Upon screening a panel of growth factor-dependent T cell lines, we found that only the cell line CT6 responded to IL-7, indeed as vigorously as to IL-2. Obviously, these findings challenge the validity of previous results on IL-2 production obtained using the CT6 cell line. However, they also demonstrate a novel and sensitive system for the bioassay of IL-7. The ability of the surveyed T cell lines to proliferate to IL-7 corresponded with the expression of IL-7 receptors (IL-7R) on the cell surface. The murine IL-7R on CT6 was shown to bind IL-7 with dual affinity and was visualized as an affinity cross-linked complex of 93 kDa. This IL-7R appears similar to that seen on murine splenic T cells and on 70Z/3, the pre-B cell line from which the murine IL-7R was cloned.