beta-Lactamases of Proteus and their role in the mechanism of the microbe resistance to penicillins and ceporin were studied. It was found that the beta-lactamase of Proteus had low activity and were produced by both beta-lactamide resistant and sensitive clinical strains of Proteus. The resistant cultures of Proteus produced enzymes more frequently (3.4--5 times) than the sensitive ones. The synthesis of beta-lactamase in the clinical Proteus strains was inducable. The high induction coefficient was achieved only in the presence of high concentrations of the inductor. No significant dependence of the culture sensitivity level of ampicillin and ceporin on the induction level was observed. The most significant part of the constitutive enzyme in Proteus was intracellular, while that of the inducable enzyme was extracellular. No correlative dependence between the culture resistance levels to penicillins and ceporin and the enzyme activity was noted. The beta-lactamase activity was not found in the transconjugants with the in vitro acquired R-factor controlling the ampicillin and ceporin resistance, as well as in the resistant mutants selected on the media with increasing concentrations of the above antibiotics. Induction of beta-lactamase synthesis was not found in these strains either. The ability of Proteus to synthesize beta-lactamase can be lost on the strain storage under laboratory conditions which was not always accompanied by reduction of the culture sensitivity to ampicillin and ceporin. The enzymatic destruction of beta-lactamides was not the main mechanism of Proteus resistance to the above antibiotics.