Lipids of Escherichia coli, Serratia marcescens and Agrobacterium tumefaciens grown in the presence of methyl-labeled methionine have been examined. A. tumefaciens produces lipids containing N-methylethanolamine, N, N-dimethylethanolamine and choline with labeled methyl groups as well as labeled cyclopropane acids. The rate of formation of methylated bases parallels growth, while the rate of cyclopropane acid formation does not. In all three organisms, cyclopropane acid formation takes place mainly in late logarithmic growth and in the stationary phase. Hydrogen as well as carbon of methionine methyl groups is incorporated into cyclopropane rings. A mutant organism blocked in methionine synthesis forms cyclopropane acids from methionine methyl groups, the extent of incorporation being undiluted by a number of known one-carbon donors.