The lac repressor protein (Lac I) is an important genetic control protein. It controls the expression of the lactose metabolizing enzymes in Escherichia coli. This regulation involves specific recognition of the operator DNA sequence (Lac O) and consequent inhibition of transcription initiation by RNA polymerase; modulation of binding at this target site by small sugar molecules, inducers, provides the basis for regulation. Different ligand binding activities are associated with the isolated domains of Lac I: specific and nonspecific DNA binding with the NH2 termini, and inducer and specific DNA binding with the core protein. This antibody could possibly be used to recover and purify protein/nucleic acid complexes (host and viral proteins associated with the viral DNA) from infected cells.