In our laboratory KES group bacteria account for about 11% of all strains isolated from in-patients and are responsible for serious infections. Their well known increasing tendency to become resistant to beta-lactams prompted us to the KES strains isolated in medical or surgical patients in 1986.87. 30 Klebsiella retrospectively review the susceptibility to antibiotics of strains out of 59 had been isolated in that period from patients previously treated with beta-lactams: no significant variation in resistance to cephalosporins, compared to the 29 strains from non treated patients, was noted. However in the treated group one strain of Klebsiella pneumoniae out of 24 showed resistance to ceftazidime and one of Klebsiella oxytoca was resistant both to cefotaxime (1 out of 16) and ceftriaxone (1 out of 12). 19 strains of Enterobacter spp. out of 32 had bees isolated from treated patients; a significant increase in resistance to 3rd generation cephalosporins (p less than 0.01), compared to the 13 strains isolated from non treated patients, was noted. Our findings suggest that it is advisable to review the in-use antibiotic policy keeping in mind the severity of the infections caused by KES group bacteria.