To isolate mutants of Escherichia coli K-12 lacking endonuclease I activity (end), a method has been developed which detects, by differential methyl green staining, undegraded deoxyribonucleic acid (DNA) in colonies previously incubated in toluene. This procedure allows isolation of mutant strains in which DNA degradation is reduced. For half of these strains, this defect has been correlated with deficiencies of endonuclease I, ribonuclease I (rns), or ribonuclease II (rne) activities. The enzymatic deficiencies of the other strains remain unknown. An rne mutation is cotransducible with serA (which is located at 56 min on the genetic map). Most end mutations, called endA, are also cotransducible with serA and are located between serA and strA. One end mutation, called endB, is located between purE and trp (i.e., between 13 and 25 min on the genetic map).