DNA transport in Streptococcus pneumoniae was studied using donor molecules labelled either at the 3' or at the 5' end, on one strand only. In contrast to 5' end label, 3' end label was not taken up by the cells indicating that entry is a polarized process. Our results together with those of previous studies are consistent with a model for entry in which double-stranded donor DNA is nicked on binding at the cell surface. Entry of a single strand then proceeds linearly from a newly formed 3' end to the extremity of the donor fragment.