The virulence of 21 clinical isolates and 10 environmental strains of Pseudomonas aeruginosa was studied by intraperitoneal challenge in mice. The LD50 values of human strains ranged from 10(7),(0) to 10(8),(5) viable cells per mouse (average: 10(7),(7)(2)). The LD50 of environmental strains ranged from 10(7),(2) to 10(8),(1) (average: 10(7),(59)). These results are not significantly different. Experimental virulence for mice of P. aeruginosa strains found in hospital environment allows no prediction on their level of pathogenicity for in-patients. Furthermore, this study proved no relationship between letal effect of P. aeruginosa for mice and following characters of strains: O serogroup, untypability and productivity of specific pigments, pyocyanin and pyorubrin.