The relationship between the inability to synthesize a complete 3-deoxy-D-manno-octulosonate region of lipopolysaccharide and cell death was investigated in a temperature-sensitive lethal mutant of Salmonella typhimurium. The defect in lipopolysaccharide synthesis is due to a mutation in the structural gene for 3-deoxy-D-manno-octulosonate-8-phosphate synthetase (designated kdsA) and results in the synthesis of a temperature-sensitive enzyme. Expression of the kdsA lesion at elevated temperatures, at which the synthesis of 3-deoxy-D-manno-octulosonate is complete blocked, is required for expression of the temperature-sensitive lethal phenotype. However, the defect in lipopolysaccharide synthesis is not alone sufficient cause for the observed cell death. Genetic evidence if presented which indicates that the mutant possesses a second mutation, or possibly multiple mutations, whose lethal expression is dependent on the inability of the mutant to synthesize a fully acylated and 3-deoxy-D-manno-octulosonate-substituted lipid A portion of lipopolysaccharide at elevated temperatures.