The Fcs receptor II (FcsRII, CD23) functions in B cell growth and differentiation and in IgE-mediated immunity. The FcsRll structure expressed on various cell types has been analyzed identifying two species, FcsRlla and FcERllb. Sequence analysis of the cloned cDNAs revealed that they differ only at the N-terminal cytoplasmic region, but share the same C-terminal extracellular region, These Fc~ Rll species appear to be generated utilizing different transcriptional initiation sites and alternative RNA splicing. FcERlla is constitutively expressed only in normal B cells and B cell lines, whereas FcERllb expression is detectable in various cell types, such as monocytes and eosinophils. Normally, FcERllb is undetectable in B cells and monocytes, and can be induced by interleukin4. Moreover, FccAllb is expressed on peripheral blood lymphocytes in atopic individuals. These findings may explain the difference in FcERlla and FccRllb function in B cells and the effector phase of IgE-mediated immunity.