A cDNA library was constructed from size-fractionated poly(A)+ RNA prepared from a murine pre-B-cell hybridoma expressing high levels of immunoglobulin heavy chain binding protein (BiP) and mu heavy chains. Transformed bacterial colonies were screened for recombinant plasmids containing cDNA coding for BiP by hybrid-selected mRNA translation. A clone, pMBiP, containing a 736-base-pair insert was shown to encode the protein. Translation in vitro of hybridoma mRNA selected by hybridization to the pMBiP cDNA yielded a single polypeptide of BiP-like size. The authenticity of this mRNA was verified by comparing the peptides obtained by the limited proteolysis of its in vitro translation product with those obtained from the in vivo produced BiP. Likewise, the authenticity of the cDNA insert was verified by an RNase A protection assay of heteroduplex molecules obtained by annealing a uniformly labeled single-strand copy of the cDNA clone with the same mRNA selected by hybridization and tested by translation. The nucleotide sequence of this clone enabled us to deduce the carboxyl-terminal 142 amino acids of BiP and to establish its kinship with the 70-kDa heat shock protein family. The finding of a single copy of the BiP gene in DNA blots of mouse and rat implies that the BiP-related RNA transcripts constitutively expressed in various murine tissues and cell lines are indeed products of the same gene. These findings imply that BiP plays a more general role than previously anticipated on the basis of the discovery of its association with immunoglobulin heavy chains.