1. Escherichia coli K-12 mutants deficient in detergent-resistant (DR) and detergent-sensi-tive (DS) phospholipases A and deficient in DS phospholipase A were isolated.
2. The growth, compositions of phospholipids and fatty acids and turnover of phospholipids of three mutants (DR^-, DS^-, DR^-DS^-) were compared with those of their parent (DR^- was isolated in a previous study).
3. Autodegradations of membrane phospholipids of 18, 000×g supernatants and precipitates of the homogenates of these three mutants were also compared with those of the parent, and the effects of various detergents and organic solvents on these activities were examined.
4. We could not identify any significant physiological role for DR or DS phospholipase A.