DNA probe tests are now available to the clinical microbiology laboratory for the rapid diagnosis of infectious disease. A unique method for DNA probe tests has been developed that allows the test to be performed in a single test tube in four basic steps within two hours. Improved assay sensitivity and specificity are achieved by targeting the DNA probes to the ribosomal RNA in bacterial cells. This new method is contrasted with conventional DNA probe procedures that involve more steps and time and other rapid diagnostic tests. Application of these new DNA probe tests are presented for Legionella, Mycoplasma pneumoniae, and the Mycobacterium avium complex in comparison with the standard diagnostic methods.