Polyclonal antibodies raised against the human erythrocyte nucleoside transporter were used to investigate the distribution of the nucleoside transporters in the placenta. Immunoblots of brush-border membranes isolated from the human syncytiotrophoblast revealed a cross-reactive species that co-migrated with the erythrocyte nucleoside transporter as a broad band of apparent M(r) 55,000. In contrast, no labelling was detected in basal membranes containing a similar number of equilibrative nucleoside transporters as assessed by nitrobenzylthioinosine (NBMPR)-binding. The absence of cross-reactive epitopes in basal membranes and their presence in brush-border membranes was confirmed by confocal immunofluorescence microscopy. These results suggest that at least two isoforms of the NBMPR-sensitive nucleoside transporter are present in the human placenta. The lumenal surfaces of fetal capillaries, small placental vessels and umbilical vein were also strongly labelled by the antibody, a finding that suggests that the high fetal-placental adenosine uptake previously reported is due to endothelial transporters.