In 1984 the European Study Group on Antibiotic Resistance (ESGAR) consecutively collected gram-negative bacilli and staphylococci blood isolates and performed susceptibility testing with 11 antibiotics using the microdilution method. In all 2,578 isolates were collected: 68% gram-negative bacilli and 32% staphylococci. The MICs of ampicillin and cefazoline for the susceptible gram-negative bacilli were 1-8 micrograms/ml; of piperacillin less than or equal to 0.5-4; of Sch 34343, cefotaxime, moxalactam, ceftazidime and aztreonam less than or equal to 0.5-2 micrograms/ml; of cefoxitin, cefuroxime and cefamandole less than or equal to 0.5-8 micrograms/ml. For susceptible staphylococci the MICs of cefazoline and cefuroxime were less than or equal to 0.5-1 micrograms/ml, and of cefoxitin, moxalactam, ceftazidime and cefotaxime, less than or equal to 0.5-32 micrograms/ml. The resistance levels varied between laboratories and countries, being lower in Northern Europe. In clinical protocols on patients with gram-negative septicemia from whom cefazoline-resistant strains were isolated, cefotaxime was the beta-lactam most commonly used (12%). In protocols on patients with staphylococcal septicemia from whom gentamicin-resistant or cefazoline-resistant strains were isolated, the most commonly used beta-lactam was cloxacillin (6%).