Endogenous fatty acid biosynthesis in the bacterium Lactobacillus plantarum is greatly decreased upon addition of exogenous fatty acids (Henderson, T.A., and McNeil, J.J. (1966) Biochem. Biophys. Res. Commun. 25, 662-669). We have demonstrated the presence of five pantothenate-containing compounds in L. plantarum which have been identified by co-chromatography with authentic samples: pantothenate, 4'-phosphopantetheine, 3'-dephosphocoenzyme A, coenzyme A, and acyl carrier protein (ACP). The concentrations of the above pantothenate-containing compounds were found to be: 0.009, 0.13, 0.067, 0.69, and 0.22 nmol/mg of protein, respectively. L. plantarum ACP was shown to have a molecular weight near that of Escherichia coli ACP but to have a lower isoelectric point (pI = 3.75). Oleate in the presence of Triton X-100 was found to reduce the concentration of ACP by 80% with little effect on the concentrations of the other pantothenate-containing compounds. The synthesis of ACP apparently ceased soon after the addition of oleate and the rate of decrease in concentration of ACP was quanitatively consistent with the previously observed rate of decrease in the initial rate of fatty acid biosynthesis in this organism (Weeks, G., and Wakil, S.J. (1970) J. Biol. Chem. 245, 1913-1921). Thus, the change in rate of fatty acid biosynthesis in L. plantarum upon addition of oleate to the medium can be quantitatively related to the concentration of ACP (and probably to the concentration of co-repressible enzymes of fatty acid biosynthesis).