The inward transport of two purines, adenosine and hypoxanthine, at 37¡C by horse erythrocytes was compared. No mediated transport of adenosine was detected in horse erythrocytes, nor was saturable, high-affinity binding of the potent facilitated-diffusion inhibitor nitrobenzylthioinosine demonstrable in horse erythrocyte membranes. In contrast, erythrocytes from most horses possessed a saturable sodium-dependent hypoxanthine transporter (apparent Km, 100 ± 28 µM; Vmax, 0·20 ± 0·08 mmol (l cells)-1 h-1; means ± S.E.M., n = 5). Guanine inhibited hypoxanthine influx (apparent Ki, 24 ± 6 µM), but adenine and xanthine had no effect. Unlike human erythrocytes, no sodium-independent hypoxanthine transporter was detected in horse erythrocytes. There are, however, a small number of animals ([similar]15 %) whose erythrocytes fail to transport hypoxanthine. This variation appears to be under genetic control, but the precise nature of the control is unknown.