In humans, two subtypes of the low-affinity Fc receptor for IgE, Fc epsilon RIIa and Fc epsilon RIIb, have been identified. The two forms differ only in a short stretch of amino acids at the cytoplasmic amino terminus. On the other hand, only one Fc epsilon RIIa-like receptor was reported in the mouse, although the existence of another subtype of transcript has been suggested. A new cDNA subtype for murine Fc epsilon RII that has amino acid homology to the human subtype b at the amino terminus was cloned by polymerase chain reaction. When expressed transiently in COS cells, the cDNA is capable of generating a functional cell surface protein that is recognized by an anti-Fc epsilon RII antibody and that has affinity to IgE. In the spleen, although the expression level is considerably lower than that of subtype a, the transcript appears in B cells and in non-B cells upon stimulation with IL-4 and LPS, in contrast to the constitutive expression of the conventional Fc epsilon RII transcript. It is also detectable in several cell lines of B- or T-cell lineages. These results suggest that this gene may be the murine counterpart of human Fc epsilon RII subtype b. Another cDNA clone for the distinct Fc epsilon RII transcript has also been isolated. The cDNA also encodes a surface protein functional on COS cells, but its human counterpart has not been found.