Interleukin (IL)-4, a pleiotropic cytokine involved in many glomerular diseases, is regulated positively by membrane-bound IL-4R (mIL-4R) and negatively by soluble IL-4R (sIL-4R). Because natural sIL-4R has been documented only in mice, we undertook this study in rats to determine whether they, too, express sIL-4R, particularly in kidney cells. A pair of IL-4R primers was designed for this purpose and used in the polymerase chain reaction. As a result, sIL-4R was found not only in rats spleen cells but also in their glomerular epithelial cells (GEC). Sequence analysis revealed that the mRNA of rat sIL-4R has a 75-bp insert sequence. This insert generated a termination TGA codon upstream from the transmembrane region, resulting in formation of the sIL-4R. Subsequent screening of the kidney cDNA library enabled us to obtain the whole 3605-bp cDNA of sIL-4R; the full-length 3530-bp mIL-4R cDNA was also identified as a much longer sequence than previously published. Among the total 39 clones positive for IL-4R, two were confirmed as sIL-4R, and 37 clones were positive for mIL-4R. Next, the translated portion of sIL-4R cDNA was constructed into an expression vector, enabling us to obtain a recombinant sIL4R-myc fusion protein. By using this recombinant sIL-4R, we proved that sIL-4R can antagonize the IL-4-induced proliferation of spleen cells. Present study demonstrated that sIL-4R is expressed in kidney cells and antagonistically functional.