We describe an in vivo approach for the isolation of proteins interacting with a protein of interest. The protein of interest is "tagged" with a portion of the biotin carboxylase carrier protein (BCCP), encoded on a specially constructed plasmid, so that it becomes biotinylated in vivo. The "query" proteins (e.g., those in a cDNA library) are tagged by fusing them to the 3' end of the lacZ gene on a lambda vector in such a way that the beta-galactosidase activity is not disrupted. These phage are transfected into cells containing the plasmid encoding the BCCP-tagged protein. The infection lyses the cells and exposes the protein complexes. The BCCP-tagged protein and any associated protein(s) are "captured" by using avidin, streptavidin, or anti-biotin antibody-coated filters. The detection of bound protein is accomplished by directly assaying for beta-galactosidase activity on the filters. Positive plaques can be plaque-purified for DNA sequencing. We have tested this approach by using c-Fos and c-Jun as our model system. We show that avidin, streptavidin, or polyclonal anti-biotin (but not a monoclonal anti-biotin) antibody is capable of specifically capturing in vivo biotinylated beta-galactosidase and c-Jun and that this capture is dependent upon the presence of both avidin and the BCCP moiety. Further, complexes containing c-Jun and c-Fos can also be isolated in this manner, and the isolation of this complex is dependent on the presence of c-Fos, c-Jun, avidin, and the BCCP moiety. We discuss the possible uses and limitations of this technique for isolating proteins that interact with a known protein.