Asthma is one of the most chronic medical conditions characterized by inflammation of the airways, mucous production, and airway hyperresponsiveness. A hallmark of asthma is the presence and activation of inflammatory cells in the airways, notably eosinophils, mast cells and T lymphocytes. Eosinophils function is related to destructive activity mediated by the toxic effects of the granule proteins and soluble mediators derived by the cells. Vitamin D, as a potent immunomodulator, has an unknown role in the eosinophil-induced asthmatic response. It was previously reported that in vivo administration of vitamin D in humans increases the expression of CC chemokine receptor 3 (CCR3) in eosinophils. However, whether or not the effect of vitamin D on eosinophils is direct or indirect is unclear. Calcitriol, the active form of vitamin D, can induce synthesis of Transforming Growth Factor (TGF-β), which is involved in airway remodeling. Eosinophils, which are recruited to the lungs by eotaxin and interleukin 5 (IL-5), are a major source of TGF-β. Therefore, the effect of calcitriol was examined on vitamin D receptor (VDR), CCR3, TGF-β1, and IL-5 receptor (IL-5R), expression in human blood eosinophils. Peripheral blood was drawn from healthy volunteers, and eosinophils, neutrophils, monocytes and lymphocytes were isolated. The purified eosinophils cells (purity >99%; viability >97%) were stimulated with calcitriol (100-300 pM and 100 nM) for 24 hours. The mRNA expression and protein expression of CCR3, VDR, TGF-β1, and IL-5R was examined by real-time PCR and Western blotting. Eosinophil chemotaxis was also assessed using Transwell migration assay and using eotaxin and IL-5 as chemokines. Also, the role of GATA-1 which is a member of GATA transcription factor family involved in eosinophil development was examined using siRNA. After calcitriol stimulation of eosinophils, VDR mRNA and protein expression were increased significantly. Eosinophils also had a significant increase in CCR3 mRNA and protein expression of eosinophils as well as a significant increase in mRNA and protein expression of TGF-β1. However, Calcitriol had no effect on IL-5R mRNA transcripts in eosinophils. Calcitriol was found to significantly enhance the migration of eosinophils in response to eotaxin, but it had no effect on the migration of eosinophils in response to IL-5. Calcitriol induced phosphorylation and increase protein expression of GATA-1. siRNA specific for GATA-1 reduced calcitriol-induced phosphorylation and increased GATA-1 protein expression, in addition, calcitriol increased protein levels of CCR3. These results demonstrate that vitamin D can directly act on human blood eosinophils to increase mRNA transcripts of CCR3, TGF-β1 and enhancing migration of eosinophils in response to eotaxin. This suggests a potential role of vitamin D in eosinophil recruitment and migration in inflammatory airways.