Restriction endonucleases show extraordinary specificity in distinguishing specific from nonspecific DNA sequences. A single basepair change within the recognition sequence results in over a millionfold loss in activity. To understand the basis of this sequence discrimination, it is just as important to study the nonspecific complex as the specific complex. We describe here the crystallization of restriction endonuclease BamHI with several nonspecific oligonucleotides, The 11-mer, 5'-ATGAATCCATA-3', yielded cocrystals with BamHI, in the presence of low salt, that diffracted to 1.9 Angstrom with synchrotron radiation. The cocrystals belong to the space group P2(1)2(1)2(1) with unit cell. dimensions of a = 114.8 Angstrom, b = 91.1 Angstrom, c = 66.4 Angstrom, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees. This success in the cocrystallization of BamHI with a nonspecific DNA provides insights for future attempts at crystallization of other nonspecific DNA protein complexes.