In 1987 and 1988 members of the European Study Group on Antibiotic Resistance collected 3440 consecutive isolates of Gram-negative bacilli (63%) and staphylococci (37%) from blood cultures and performed susceptibility testing by the microdilution method. The MICs of ampicillin and cefazolin for susceptible Gram-negative bacteria were 1–8 mg/l, of piperacillin ≤ 0.5–4 mg/l, of aztreonam, imipenem, cefotaxime and ceftazidime ≤ 0.125–1 mg/l and of ciprofloxacin ≤ 0.125–0.5 mg/l. For susceptible staphylococci the MICs of cefazolin were ≤ 0.5–8 mg/l, of cefotaxime 1–4 mg/l, of ceftazidime 4–16 mg/l, of imipenem and ciprofloxacin ≤ 0.125–1 mg/l. The antibiotic resistance rates varied between laboratories, being generally lower in northern Europe, except for imipenem, which showed uniform, low resistance rates. In Escherichia coli resistance to ampicillin, piperacillin and cefazolin could be seen to have increased since a previous survey.