Interleukin-5 (IL-5) is a hematopoietic differentiation factor that promotes the development of mature eosinophils from progenitors in bone marrow. We present a multifactorial microarray study documenting the transcriptional events in bone marrow of wild-type and IL-5-deficient mice at baseline and in response to infection with Schistosoma mansoni. The microarray data were analyzed by a 4-way subtractive algorithm that eliminated confounding non-IL-5-related sequelae of schistosome infection as well as alterations in gene expression among uninfected mice. Among the most prominent findings, we observed 7- to 40-fold increased expression of transcripts encoding the classic eosinophil granule proteins (eosinophil peroxidase, major basic protein, the ribonucleases) together with arachidonate-15-lipoxygenase and protease inhibitor plasminogen activator inhibitor 2 (PAI-2), in the IL-5-producing, infected wild-type mice only. This was accompanied by increased transcription of genes involved in secretory protein biosynthesis and granule-vesicle formation. Interestingly, we did not detect increased expression of genes encoding eosinophil-related chemokine receptors (CCR1, CCR3) or members of the GATA or CCAAT/enhancer binding protein (C/EBP) transcription factor families. These data suggest that the IL-5-responsive progenitors in the mouse bone marrow are already significantly committed to the eosinophil lineage and that IL-5 promotes differentiation of these committed progenitors into cells with recognizable and characteristic cytoplasmic granules and granule proteins.