We evaluated the levels of mRNAs encoding cationic proteins in peripheral blood eosinophils (PBE) purified from patients with eosinophilia and in eosinophils differentiated from cord blood cells (CBC) by culture with recombinant human interleukin-3 (rhIL-3), rhGM- CSF, and rhIL-5. Messenger RNAs encoding eosinophil peroxidase (EPO), major basic protein (MBP), eosinophil-derived neurotoxin (EDN), and eosinophil cationic protein (ECP) were detected by Northern blot hybridization with the respective specific oligonucleotide probes. In mature PBE, MBP mRNA appeared to be absent, whereas EPO mRNA was barely detectable in only 5 of the 19 patients. In contrast, EDN and ECP mRNAs were observed in the PBE of all patients. In CE, EPO, and MBP, mRNAs were abundant in immature eosinophils and their amounts decreased after differentiation toward eosinophils. ECP and EDN mRNAs followed the same patterns, but mRNAs were less abundant at all timepoints studied. Study of mRNA t1/2 during the time course of differentiation indicated that changes in the stability of the different mRNAs were not responsible for the variations observed in the steady-state levels. Together, these results suggest that regulation of expression differs among EPO, MBP, EDN, and ECP mRNAs during the time course of eosinophil differentiation.