Murine Ba/F3 cells were transfected with cDNA for the alpha-chain of the murine interleukin-5 (IL-5) receptor and cloned lines of these cells were able to proliferate in response to as little as 2.5 pg/ml of IL-5. The bioassay was demonstrated to be specific for IL-5 and was able to measure IL-5 produced in culture by organs from adult C57BL/6 and BALB/c mice. The highest levels of IL-5 were produced by lung tissue but thymus and bladder consistently produced IL-5 and more variable production was observed by the heart, spleen, muscle, bone shaft, uterus and testes. Bone marrow cells produced no detectable IL-5. Observed levels of production of IL-5 were similar when using organs from mice lacking high-affinity receptors for IL-5 and from nu/nu, RAG-1-/- and NOD/SCID mice lacking T lymphocytes. In inflammatory peritoneal exudates induced by the injection of casein plus bacteria, levels of induced IL-5 were higher if the mice lacked high-affinity receptors for IL-5. The data indicate that T lymphocytes are not the dominant cellular source of IL-5 in organ-conditioned media and that local IL-5 production can occur with a wide range of normal murine organs.