We obtained two R plasmids, i.e., Rms195 and Rms298, from a clinical isolate, E. coli GN5503. Penicillin β-lactamase (PCase) was extracted from ML 1410 Rms195+ and Rms298+, and was purified by chromatography. Rms195 PCase was identical to the type I PCase mediated by R-TEM, RI and Rms212. The isoelectric point of Rms298 PCase was 5.9 and its molecular weight was 21, 000±1, 000. The substrate profile and physicochemical properties indicate that Rms298 PCase belongs to the type IV PCase mediated by Rms139 isolated from Pseudomonas aeruginosa.