The ribosomal differentiation of medical micro-organisms (RIDOM) web server, first described by Harmsen et al. [Harmsden,D., Rothganger,J., Singer,C., Albert,J. and Frosch,M. (1999) Lancet, 353, 291], is an evolving electronic resource designed to provide micro-organism differentiation services for medical identification needs. The diagnostic procedure begins with a specimen partial small subunit ribosomal DNA (16S rDNA) sequence. Resulting from a similarity search, a species or genus name for the specimen in question will be returned. Where the first results are ambiguous or do not define to species level, hints for further molecular, i.e. internal transcribed spacer, and conventional phenotypic differentiation will be offered ('sequential and polyphasic approach'). Additionally, each entry in RIDOM contains detailed medical and taxonomic information linked, context-sensitive, to external World Wide Web services. Nearly all sequences are newly determined and the sequence chromatograms are available for intersubjective quality control. Similarity searches are now also possible by direct submission of trace files (ABI or SCF format). Based on the PHRED/PHRAP software, error probability measures are attached to each predicted nucleotide base and visualised with a new 'Trace Editor'. The RIDOM web site is directly accessible on the World Wide Web at http://www.ridom.de/. The email address for questions and comments is email@example.com.