Glucocorticoid (GC) insensitivity is a challenging clinical problem associated with many chronic inflammatory disorders and life-threatening disease progression. The molecular basis of GC insensitivity, however, is unknown. Alternative splicing of the GC receptor (GCR) pre-mRNA generates a second GCR, termed GCRbeta, which does not bind GC but antagonizes the transactivating activity of the classic GCR, termed GCRalpha. GC-insensitive conditions have been associated with increased GCRbeta expression. Whether or not increased GCRbeta expression can contribute to GC insensitivity, however, remains controversial. To more precisely demonstrate the effect of GCRbeta on steroid responsiveness, we virally transduced GCRbeta cDNA into mouse DO-11.10 hybridoma cells, as mice are known to be deficient in the GCRbeta gene. We demonstrate that viral transduction of GCRbeta cDNA into mouse hybridoma cells to induce stable expression of GCRbeta results in GC insensitivity of these cells. Furthermore, in such cells GCRalpha is complexed with GCRbeta. Such heterodimer formation may account for the reduced effectiveness of GC action in cells overexpressing GCRbeta.