Henderson, Thomas O. (North Carolina State University, Raleigh), John J. McNeill, and S. B. Tove. Folic acid involvement in cyclopropane fatty acid synthesis in lactobacilli. J. Bacteriol. 90:1283-1287. 1965.-Evidence for the involvement of folic acid in the synthesis of lactobacillic acid (11,12-methyleneoctadecanoic acid) was obtained with Lactobacillus plantarum and L. casei. L. plantarum, in the presence of aminopterin, incorporated 25% less radioactivity from methionine-methyl-C(14) into long-chain fatty acids than did control cultures, whereas the incorporation of acetate-1-C(14), representing overall fatty acid synthesis, was the same in both conditions. Control cultures of L. plantarum contained 3 to 4.5 times more lactobacillic acid than did aminopterin-grown cultures. Similarly, L. casei, in the presence of folic acid, synthesized six times more lactobacillic acid than did folic-deficient cultures. The results of this study indicate that coenzymes of the folic acid group are involved in the synthesis of lactobacillic acid in these organisms.