The polymeric guanidine derivatives have long proven to be efficient disinfectants and antiseptics. They have clearly expressed bactericidal, virucidal, fungicidal, and algicidal properties [1,2]. There is much information on the application of polymeric guanidine derivatives in the composition of different disinfectants and antiseptics, for instance, e.g., in the treatment of hospital-acquired infections (biofilms) [2,3,4]. In addition, polyhexamethylene guanidine (PHMG), has also been used with some effect in the purification of sewage and underground waters [5].We conducted comparative studies of the bactericidal properties of two polymeric derivatives of guanidine together with private firm «Termit» (Rivne, Ukraine).We studied and compared antimicrobial effects of polyhexamethylene guanidine (PHMG) and polyhexamethylene biguanidine (PHMB) both in the hydrochloride form. They differ somewhat in price and disinfectant properties.We used three test strains of microorganisms. These were gram-negative bacteria Esherichia coli АТСС № 25922 and Pseudomonas aeruginosa АТСС № 27853 (F 51), and gram-positive bacteria Staphylococcus aureus АТСС № 25923 (F 49). Research was conducted using generally accepted methods [6,7,8].Aqueous solutions of drugs were prepared in concentrations of 0.01-1.0%, pH 5.5-6.0.For E. coli, the minimum bactericidal concentration (MBС) of PНMG was 0.05% for a duration of exposure of 30 min., for 15 min. exposure, a weak growth of colonies was still observed. MBC for PHMB was 0.02% in 15 min. Exposure.The situation was similar for P. aeruginosa. MBС of PНMG was 0.05% for a duration of exposure of 5 min., for PHMB it was 0.02%.For S. aureus MBС of PНMG was 0.04% for 15 min. exposure, MBС for PHMB for the same time was also 0.02%. Therefore, PHMB shows greater efficiency than PНMG in vitro.At the same time, our further studies showed that in the presence of a protein load (cattle serum), the advantages of PHMB almost disappear, no significant differences in the action of both disinfectants are observed.