Interleukin 7 (IL-7) is a cytokine initially referred to as pre-B cell growth factor because of its growth-promoting activity for cells early in the B lymphocyte lineage. Subsequent to IL-7 gene cloning and expression, IL-7 was also found to influence the activity of cells in the T lineage. Some of the effects of IL-7 on B and T lymphocyte behavior, both in vitro and in vivo, are discussed here. The growth of pre-B cells in Whitlock-Witte long-term bone marrow cultures [1] provided the basis for the cloning of IL-7 [2]. Such cultures consist of at least two cellular elements: 1) a stromal cell layer and 2) lymphoid cells growing in clusters around the stromal cells. A cDNA encoding IL-7 was cloned utilizing a direct expression cloning strategy. A transformed stromal cell line was used as a source of RNA from which a cDNA library was constructed, inserted into a mammalian expression vector and expressed in COS 7 cells. The COS cells were then screened for secretion of IL-7 into the culture medium as defined by the ability to support stromal cell-independent growth of the lymphoid elements from long-term Whitlock-Witte cultures. In addition to its effects on cells from long-term marrow cultures, IL-7 stimulates growth of cells in cultures of fresh bone marrow [3]. Cells from whole bone marrow plated in semisolid agar form colonies in the presence of IL-7. If the marrow is first enriched for cells expressing the B220 antigen before culture in IL-7, the plating efficiency is increased dramatically.(ABSTRACT TRUNCATED AT 250 WORDS)