The Fcε receptor II (FcεRII, CD23) functions in B cell growth and differentiation and in IgE-mediated immunity. The FcεRII structure expressed on various cell types has been analyzed identifying two species, FcεRIIa and FcεRIIb. 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 FcεRII species appear to be generated utilizing different transcriptional initiation sites and alternative RNA splicing. FcεRIIa is constitutively expressed only in normal B cells and B cell lines, whereas FcεRIIb expression is detectable in various cell types, such as monocytes and eosinophils. Normally, FcεRIIb is undetectable in B cells and monocytes, and can be induced by interleukin-4. Moreover, FcεRIIb is expressed on peripheral blood lymphocytes in atopic individuals. These findings may explain the difference in FcεRIIa and FcεRIIb function in B cells and the effector phase of IgE-mediated immunity. © 1988.