Tacrolimus is a macrolide immunosuppressant used to prevent graft rejection in transplant patients and also in the treatment of atopic dermatitis. It inhibits T-lymphocyte activation by preventing transcription of gene encoding cytokines involved in the activation of the immune response. The drug has also been observed in the inhibition of eosinophil and neutrophil migration. We studied its immunosuppressant effects upon eosinophil viability and activation by monitoring CD 69+ (an activation marker) mean fluorescence and expression. Whole blood was collected from eosinophilia patients and separated for white blood cells through a 75% percoll gradient. All cultures were set at a concentration of 1.0 × 10^6 cells/mL and treated with IL-5 (1 ng/mL). Drug was separately added to specific culture tubes in order to study the differing effects on cell activation. Cytokine and drug were added bi-weekly to maintain appropriate culture concentrations. Flow cytometry was performed in 4, 7, and 14 day increments to monitor eosinophil activation through CD 69 expression and fluorescence. Four day incubation with IL-5 alone showed 72% of eosinophils staining positive for CD 18+/69+. When 50 ng/mL of tacrolimus was present, 63% of the cells were positive for CD 18+/69+ and at 500 ng/mL of tacrolimus, 66% of cells stained positive for both markers. By 14 days of incubation, 64% of eosinophils stained for CD18+/69+ and this decreased to 45% with 50ng/mL of drug and to 41% with 500ng/mL. Mean fluorescence for cells staining for CD69+ declined in activation with increasing concentrations of tacrolimus. Thus, tacrolimus may decrease eosinophil activation which may contribute to its anti-inflammatory effects.