To study in standardbred horses interindividual variation in the influx of lactate into red blood cells, venous blood samples were collected from 89 horses from 2 wk to 9 yr of age. For 62 horses, the rate of influx was normally distributed with a mean rate of 4.09 nmol.mg protein-1.min-1 at a lactate concentration of 10 mM, and the respective value for the other 27 horses was 0.58 nmol.mg protein-1.min-1. At 30 mM of lactate, the rates were 8.71 and 1.97 nmol.mg protein-1.min-1, respectively. This bimodal distribution was independent of age. In horses with high transport activity, the monocarboxylate transporter (MCT) appears to be the major carrier, whereas, in those with low transport activity, no activity of the MCT could be detected. The band 3 protein may account for 18-39% of transport activity. With all age groups combined, the transport activity tended to be higher in mares than in stallions. Lactate transport into red blood cells seems thus to be an inherent property in which participation of various transporters varies interindividually.