A series of positional isomers of trans-octadecenoic acid were tested for their ability to support growth of microbial mutants that could not synthesize unsaturated fatty acids. The bacterial strain used (a variant of Escherichia coli 30E) grew with supplements of the trans isomers only at high temperatures (38 degrees) and with acids containing the trans-ethylenic bond between carbon atoms 8 through 13. The yeast mutant (Saccharomyces cerevisiae KD46) grew only with the 9-trans-octadecenoate giving cell yields about one-fifth those obtained with oleate. Although the trans isomers had little effect on the growth of the bacteria in the presence of oleate, they inhibited the growth of yeast with oleate. Inhibition was strongest for the 4, 6, 7, 11, and 12 isomers, almost negligible for the 8 isomer and of differing intermediate degrees for the others. The inhibitory effects had no correlation with the melting points of the acids and appeared to reflect selective action(s) on the metabolism of the cell. When the net yield of the yeast cultures with oleate was lowered by the effect of added trans acids, there was a marked accumulation of triglycerides and nonesterified acids in the cells. The marked increase in triglyceride content while phospholipid per cell remained relatively constant suggest that trans acids, in addition to forming inadequate membrane lipids, may also interfere with a basic control point in lipid metabolism.