An energy source (glucose or lactose) was required for the transport of L-glutamic acid by Lactobacillus helveticus. Mg(2+), K(+) and Li(+) increased its accumulation while Ca(2+) and Na(+) decreased it. It was inhibited by NaF, indicating that ATP may be involved in uptake. Optimum transport was at pH 7.3 and 45°C. L-Glutamic acid transport showed a high degree of stereospecificity, as neither D-glutamate nor D-aspartate were active. Proton-conducting uncouplers, like carbonyl cyanide-m-chlorophenylhydrazone, and ionophores (nigericin, monensin and gramicidin) were strongly inhibitory. These results indicate that a proton motive force may be involved in the transport of L-glutamic acid.