Some of the present problems in ribotyping are associated with a lack of uniform reactivity of probes when bacterial DNAs are of phylogenetically diverse origins. To overcome these problems, a set of five oligonucleotides (referred to as OligoMix5) was selected to react with conserved sequences located near both extremities of rrs (16S rRNA gene) and near both extremities and the middle of rrl (23S rRNA gene). DNA samples from 13 bacterial species selected to represent various phylogenetic branches within the Eubacteria were cleaved by a restriction endonuclease and electrophoresed in 0.8% agarose, and the fragments were vacuum-transferred to nylon membranes and hybridized with digoxigenin-labelled OligoMix5, plasmid DNA from pKK3535 (cloned rrn operon from Escherichia coli) or pBA2 (cloned rrs from Bacillus subtilis), or acetylamino-fluorene-labelled E. coli 16 + 23S rRNA. The results showed OligoMix5 to visualize patterns in DNA from phylogenetically diverse bacteria with comparable intensity. Banding patterns (not band intensity) obtained with OligoMix5 were identical with those obtained with 16 + 23S rRNA or plasmid pKK3535 for each strain studied and represented complete ribotypes. For DNA from Gram-positive bacteria, complete ribotypes were observed after prolonged enzymatic detection of bands when probes were either E. coli 16 + 23S rRNA or pKK3535. Patterns given by plasmid pBA2 were subsets of the complete ribotypes for 9/13 strains. Each oligonucleotide of the OligoMix5 set was used as a probe to determine its contribution to the complete ribotype. The five oligonucleotide probes, used individually, visualized one to four patterns per DNA sample. Use of DNA from Xenorhabdus sp. CIP 105189 cleaved by EcoRI is suggested to control the quality of the oligonucleotide probes composing OligoMix5. Probe OligoMix5 was found to be an essential tool for ribotyping phylogenetically diverse eubacteria.