Lactate transport is mediated in most tissues by H+-monocarboxylate-- cotransporters (MCTs). We have cloned and sequenced the lactate transporter from Ehrlich Lettré tumour cells by using the polymerase chain reaction (PCR) to amplify MCT1-related sequence from cDNA. The sequence is 93% and 87% identical to MCT1 from Chinese hamster and human respectively and so represents mouse MCT1. Most differences between MCT1 from Chinese hamster and mouse are conservative substitutions, located in hydrophilic parts of the molecule. Specific antipeptide antibodies confirm the presence of MCT1 protein in membranes from Ehrlich Lettré tumour cells. One difference between the mouse and Chinese hamster MCT1 is the absence of a predicted external consensus sequence for N-linked glycosylation in the mouse sequence. Using N-glycanase-F treatment and an in vitro translation system, we provide evidence that this glycosylation site is not actually utilised in Chinese hamster MCT1. These results are discussed in relation to current understanding of the roles of glycosylation of membrane proteins.