Oligonucleotide probes specific for detection of the TEM-1 and OXA-1 beta-lactamase genes were compared with isoelectric focusing in 114 gram-negative beta-lactamase-producing strains representing at least 16 species. Correlations of 96 and 100% with isoelectric points were found for the TEM-1 and OXA-1 probes, respectively.