Background: IL-5 plays a central role in eosinophil and basophil differentiation, exerting its effects through the IL-5 receptor (IL-5R⍺). Currently, little is known concerning regulation of IL-5R⍺ expression in the context of commitment of hemopoietic progenitor cells to the eosinophil and basophil lineages. Objective: Because all-trans retinoic acid (ATRA) is known to modulate some aspects of hemopoietic differentiation, we examined the effects of ATRA on eosinophil-basophil differentiation and IL-5R⍺ expression. Methods: Progenitor cells were obtained from bone marrow aspirates and cord blood samples. Enriched populations of CD34⁺ cells were isolated by means of positive immunomagnetic selection with MACS beads. Results: In semisolid methylcellulose cultures of normal human bone marrow, ATRA (10⁻⁶ mol/L) selectively suppressed eosinophil-basophil colony-forming units but had no effect on granulocyte-macrophage colony-forming units. Similarly, ATRA (10⁻⁶ mol/L) inhibited eosinophil-basophil differentiation of cord blood CD34⁺ cells in liquid culture, whereas neutrophil differentiation proceeded without impediment. Most importantly, these effects of ATRA (10⁻⁸ to 10⁻⁶ mol/L) on CD34⁺ cells were associated with a dose-dependent inhibition of IL-5R⍺ but no change in GM-CSF receptor expression, as detected with flow cytometry. Conclusions: These findings indicate that retinoids can differentially regulate expression of IL-5R⍺, but not GM-CSF receptor, and that these effects have functional consequences in vitro on eosinophil and basophil differentiation. (J Allergy Clin Immunol 2002;109:307-13.)