Lineage-specific hematopoietins apparently act in concert with multipotent factors in an orderly sequence of growth and differentiation. We have used the human acute promyelocytic leukemia cell line HL-60 to examine basophilic differentiation, using radioenzymatic assay of histamine content as an end point. Recombinant human interleukin 1 (rhIL-1), rhIL-2, rhIL-4, and recombinant human alpha and gamma interferons did not stimulate basophilic differentiation either in the presence or absence of sodium butyrate, an important cofactor for induction of differentiation. In contrast, rhG-CSF (granulocyte colony-stimulating factor), rhGM (granulocyte-macrophage) CSF, rhIL-3, rhIL-5, nerve growth factor, conditioned medium (CM) from the hairy T cell leukemic line Mo, and nasal polyp epithelial CM stimulated significant increases in histamine content in HL-60 cells at day 5 in vitro. GM-CSF did not account for all of the basophilic differentiating activity in Mo-CM. The data suggest that a unique, lineage-specific, basophilic cell differentiation factor is produced by T cells and point to the possible diagnostic and therapeutic relevance of in situ hematopoietic mechanisms in human respiratory disease.