Mutations, sulA and sulB, that suppress the UV sensitivity conferred by the lon mutation have been isolated and precisely positioned on the linkage map of Escherichia coli. The E. coli B strains Bs-3 and Bs-8 have been shown to possess sulA mutations. Also the E. coli K12 strain J6271 that possesses a suppressor of the lon mutation, previously designated as suf, has been shown to be a sulA mutation. A series of methylmethane sulphonate resistant derivatives of an E. coli K12 lon strain has been isolated and genetically characterized. In addition to sulA mutations, a second suppressor sulB was identified and located between leu and azi genes on the chromosome. Neither sulA or sulB mutations result in increased sensitivity to the antibiotics ampicillin, rifampicin, or actinomyein D, nor do they have any significant effect upon the overproduction of mucopolysaccharide caused by the lon mutation. Under some growth conditions the sulB mutation causes cells to be temperature sensitive for the cell division process at 42 °C.