Thymic stromal lymphopoietin (TSLP) is a novel cytokine that was found to promote the development of murine B cells in vitro. This protein, which is expressed in a number of tissues including heart, liver and prostate, prevented apoptosis and stimulated growth of the human acute myeloid leukemia (AML)-derived cell line MUTZ-3. Anti-interleukin (IL)-7 receptor antibodies (Abs) neutralized this effect indicating that TSLP binds to at least part of the IL-7 receptor complex. TSLP induced phosphorylation of signal transducer and activator of transcription (STAT)-5.