Human CD23 (the low affinity IgE receptor) is a B cell differentiation marker involved in inflammatory responses. Two isoforms (CD23a and CD23b) are known, which differ only in their cytoplasmic domain. Whereas CD23b expression is specifically induced by IL-4 on B cells and cells of the myeloid lineage, CD23a expression is restricted to B cells. Each isoform is regulated by its own promoter. Pax-5 is a B-cell-restricted transcription factor with an essential role in early and late B cell development. Analyses of the CD23a promoter revealed a Pax-5-binding site, which can compete a high affinity Pax-5-binding site or directly bind Pax-5 protein in electrophoretic mobility shift assays. Introducing mutations into this site abrogates the binding. Expression of Pax-5 in 293 cells resulted in a seven- to tenfold activation of a CD23a core promoter construct. Most importantly, ectopic expression of Pax-5 in the monocytic cell line U-937, which regularly expresses only the CD23b isoform, led to CD23a expression after stimulation with IL-4 and PMA. Our results suggest that Pax-5 is a key regulator of the B-cell-restricted expression of the CD23a isoform.