Among non-B, non-T cells in the spleen and among unfractionated bone marrow cells, there is a population of cells that are capable of producing IL-4 in response to cross-linkage of FcεRI or FcγRII. Their IL-4-producing capacity is strikingly enhanced by treatment of the cells or of the animals donating such cells with interleukin-3 (IL-3). FcεR+ cells constitute 1–2% of splenic non-B, non-T cells and of bone marrow cells from normal donors but they contain all the capacity to produce IL-4 in response to cross-linkage of FcεRI or FcγRII or to treatment with ionomycin. FcεR- cells fail to make such responses. Electron-microscopic analysis indicates that virtually all the granulated or vacuolated FcεR+ cells are of the basophil lineage. However, it has not yet been resolved whether these cells or immature mast cells, presumably in the FcεR+ granule/vacuole-negative cell population, are the principal producers of IL-4 in response to thes