Lambda DNA, a linear, double-stranded phage DNA containing 12 bp single-stranded complementary 5'-ends, is derived from an Escherichia coli bacteriophage (Bacteriophage lambda cI857 Sam7). Lambda DNA (48,502 bp) may be used as a molecular weight size marker during nucleic acid gel analysis following digestion with a restriction enzyme (such as HindIII). Lambda DNA can also be used as a substrate in restriction enzyme activity assays.