Ehrlich ascites-tumour cells were investigated with regard to their stability to transport L-lactate by measuring either the distribution of [¹⁴C]lactate or concomitant H⁺ ion movements. The movement of lactate was dependent on the pH difference across the cell membrane and was electroneutral, as evidenced by an observed 1:1 antiport for OH^- ions or 1:1 symport with H⁺ ions. 2. Kinetic experiments showed that lactate transport was saturable, with an apparent Km of approx. 4.68 mM and a V_max. as high as 680 nmol/min per mg of protein at pH 6.2 and 37°C. 3. Lactate transport exhibited a high temperature dependence (activation energy = 139 kJ/mol). 4. Lactate transport was inhibited competitively by (a) a variety of other substituted monocarboxylic acids (e.g. pyruvate, K_i = 6.3 mM), which were themselves transported, (b) the non-transportable analogues α-cyano-4-hydroxycinnamate (K_i = 0.5 mM), α-cyano-3-hydroxycinnamate (K_i = 2mM) and DL-p-hydroxyphenyl-lactate (K_i = 3.6 mM) and (c) the thiol-group reagent mersalyl (K_i = 125 muM). 5. Transport of simple monocarboxylic acids, including acetate and propionate, was insensitive to these inhibitors; they presumably cross the membrane by means of a different mechanism. 6. Experiments using saturating amounts of mersalyl as an “inhibitor stop” allowed measurements of the initial rates of net influx and of net efflux of [¹⁴C]lactate. Influx and efflux of lactate were judged to be symmetrical reactions in that they exhibited similar concentration dependence. 7. It is concluded that lactate transport in Ehrlich ascites-tumour cells is mediated by a carrier capable of transporting a number of other substituted monocarboxylic acids, but not unsubstituted short-chain aliphatic acids.