Building a map of restriction sites from double-digest gel data can be a complex and frustrating task, especially when many DNA fragments are detected or when the gel results are ambiguous. 'Double Digester' is an interactive, graphical computer program which helps researchers understand and resolve such data. It explicitly represents the experimental data, the associated uncertainties, the researcher's hypotheses and possible map interpretations. Alternative solutions are frequently possible, and the differences between them may help determine which additional experiments might resolve ambiguities. Initial use has confirmed the benefits of this approach, and has suggested ways in which it can be refined and extended. Double Digester meets the need for a practical tool to help build restriction maps, and also illustrates how a computer-based tool can confront experimental uncertainty in an integrated fashion.