We have previously shown that certain bone marrow-derived mast cell (BMMC) lines proliferate in response to a mast cell growth-enhancing activity (MEA) that is distinct from interleukin (IL) 3 and IL 4. Here we provide evidence that MEA is identical with the recently cloned mouse T cell growth factor P40. The evidence is as follows: (a) recombinant P40 displayed all the biological activities ascribed to MEA: it supported the growth of MEA-sensitive BMMC lines, it induced IL 6 secretion by these cells, and it enhanced survival of primary mast cell cultures; (b) highly purified MEA stimulated the growth of P40-dependent cell lines; (c) a rabbit monospecific antiserum directed against P40 specifically inhibited the action of MEA on BMMC; (d) specific binding sites for P40 were detected on BMMC and (e) MEA competed with P40 for binding to P40-dependent T cells, indicating that the two molecules interact with the same receptor. These observations further extend the range of biological activities ascribed to P40 and warrant its proposed designation as IL 9.