Ten clinical isolates of Proteus mirabilis were found to produce an inhibitor-resistant TEM beta-lactamase (IRT) in association with a TEM-1 enzyme. The IRT enzyme was derived from TEM-2. The blaIRT gene differs from blaTEM-2 gene by one point mutation which leads to the amino-acid substitution Arg-->Ser at position 244, as observed for the original IRT-2 enzyme derived from TEM-1 reported in Escherichia coli. This is the first report of an IRT beta-lactamase derived from TEM-2.