The web server probeCheck, freely accessible at http://www.microbial-ecology.net/probecheck, provides a pivotal forum for rapid specificity and coverage evaluations of probes and primers against selected databases of phylogenetic and functional marker genes. Currently, 24 widely used sequence collections including the Ribosomal Database Project (RDP) II, Greengenes, SILVA and the Functional Gene Pipeline/Repository can be queried. For this purpose, probeCheck integrates a new online version of the popular ARB probe match tool with free energy (DeltaG) calculations for each perfectly matched and mismatched probe-target hybrid, allowing assessment of the theoretical binding stabilities of oligo-target and non-target hybrids. For each output sequence, the accession number, the GenBank taxonomy and a link to the respective entry at GenBank, EMBL and, if applicable, the query database are displayed. Filtering options allow customizing results on the output page. In addition, probeCheck is linked with probe match tools of RDP II and Greengenes, NCBI blast, the Oligonucleotide Properties Calculator, the two-state folding tool of the DINAMelt server and the rRNA-targeted probe database probeBase. Taken together, these features provide a multifunctional platform with maximal flexibility for the user in the choice of databases and options for the evaluation of published and newly developed probes and primers.