A total of 451 extraintestinal, clinically relevant strains of the Klebsiella, Enterobacter and Serratia spp. isolated over a nine-month period from hospitalized patients at four different centers in Italy were investigated. Identification using the API 20E system showed that isolates belonged to 12 different species. Overall, strains of Klebsiella, Enterobacter and Serratia were in a ratio of approximately 3.4:2:1. Fifty-nine per cent of all strains were from urinary specimens, 12% from respiratory secretions, 10% from wounds and abscesses, and lower percentages from other sources. All strains were tested for their susceptibility to ten antibiotics. The rate of resistance to most drugs was generally greater in Enterobacter and Serratia than in Klebsiella. The overall incidence of strains of the intermediate category (i.e. between full sensitivity and resistance) was unexpectedly high. Both the relative frequency and the antibiotic susceptibility of strains of the various species varied from center to center; possible reasons for such differences are examined.