Duplex DNA is isolated from bacteriophage lambda (cI857ind 1 Sam 7). The bacteriophage is grown in an E. coli host which is deficient in adenine methylase (dam-). Therefore, those deoxyadenosine positions which are methylated in a wild type E. coli host are not methylated in the dam- host. This DNA gives complete digests with restriction endonucleases that are sensitive to dam methylation. Lambda DNA (N6-methyladenine-free) is 48,502 base pairs in length.