We have constructed expression plasmids containing the genes for the MuLV and the HIV integration proteins. When introduced intoEscherichia coli, these plasmids cause the production of proteins of the expected molecular weight (43K for MuLV, 31 K for HIV). The wild-type MuLV coding region induces the synthesis of large amounts of integration protein; to obtain large amounts of the full-length HIV integration protein inE. coli, it was necessary to modify the coding region to disrupt a sequence that promotes efficient internal translational initiation inE. coli. It is possible to disrupt this sequence without altering the encoded amino acids; the modified plasmid makes large amounts of the full-length protein and small amounts of the internally initiated protein. Both the MuLV and HIV integration proteins require SDS to be solubilized inE. coli extracts; however, following solubilization with SDS and transfer to a nitrocellulose filter, both integration proteins bind DNA.