We have demonstrated that an immature prebasophilic cell line,KU812 cells can be induced to differentiate into basophil-like cells when cultured with hydrocortisone (HC) with enhanced cell surface expression of FcepsilonRI, a high affinity IgE receptor. In this study, we report that sodium nitroprusside (SNP), an intracellular NO donor, also induces cell surface expression of FcepsilonRI on KU812 cells. Cell surface FcepsilonRI expression was detected in about 20% of KU812 cells treated with SNP for 14 days as well as the cells treated with HC for 7 days, while non-treated KU812 cells did not express FcepsilonRI on their cell surface. However, Wright-Giemsa staining and flowcytometry analysis of CD13 and CD15 antigens on HC and SNP treated KU812 cells demonstrated that SNP induced eosinophilic differentiation in KU812 cells differently from HC which induced basophilic differentiation. To further confirm this result, we performed RT-PCR against mRNAs specific for eosinophils, such as eosinophil-derived neurotoxin (EDN) and eosinophil peroxidase(EPO). SNP treated KU812 cells but not HC treated cells expressed EDN and EPO mRNA depending upon the induction of differentiation,clearly demonstrating that SNP induces eosinophilic differentiation in KU812 cells. To clarify that different signaling cascades were activated in HC and SNP treated KU812 cells, we analyzed activities of AP-1, NF-AT and NF-kappaB transcription factors by EMSA, which are known to be involved in signal transduction pathways downstream from the FcepsilonRI molecule of basophils. All these three transcription factors were activated in HC treated KU812 cells,but not in non-treated and SNP treated KU812 cells. These results indicate that KU812 cells are multi-potent precursor cells which can be induced to differentiate into basophils and eosinophils upon exogenous signals, and that NO is an important factor to decide the eosinophilic differentiation in KU812 cells with enhanced surface expression of FcepsilonRI, and further suggest that different signaling cascades can be activated between basophilic and eosinophilic differentiation in KU812 cells.