A total of 503 clinical strains of Ps. aeruginosa isolated in hospitals of 4 towns in 1976 - 1978 were studied. It was shown that a significant number of the isolates were resistant to high concentrations of streptomycin (83.3 per cent), kanamycin (71 per cent), sulfanilamide (70.2 per cent) and mercuric chloride (61.6 per cent). Strains resistant to gentamicin (29.8 per cent) and carbenicillin (20.2 per cent) occurred comparatively rare. Resistance to polymyxin and rifampicin was recorded in 2 and 4 per cent f the isolates respectively. It was found that 77 per cent of the isolates could transfer on conjugation the resistance determinants to the polyauxotrophic strains of Ps. aeruginosa ML 4262 (PAO). 83 conjugative plasmids controlling resistance to streptomycin (85.5 per cent), tetracycline (20.5 per cent), chloramphenicol (30.1 per cent), gentamicin (18.0 per cent), kanamycin (7.2 per cent), carbenicillin (13.7 per cent), sulfanilamides (125.3 per cent), organic and inorganic mercury compounds, hydroxyanions of chromium, tellurium and boron and UV radiation were isolated. The frequency of the plasmid transfer ranged from 10(-1) to 10(-6) (per donor cell). 4 nonconjugative plasmids were isolated. 2 of them determined resistance to streptomycin and sulfanilamide and 2 resistance to streptomycin, sulfanilamide and carbenicillin. The molecular mass of the plasmids was within 5.5 to 280 Md. The majority of the conjugative plasmids were classified as belonging to the incompatibility group P-2, the others belonged to groups P-1, P-3, P-5 and P-7. The nonconjugatiplasmids belonged to the incompatibility group P-4.