The activity of azithromycin against 225 clinical strains of Acinetobacter baumannii isolated consecutively from 26 Spanish hospitals in November 2000 was studied. The MICs of azithromycin were determined by microdilution, according to the NCCLS guidelines. The bactericidal activity of azithromycin against 15 clonally unrelated A. baumannii strains with different antimicrobial susceptibility patterns was tested using the subculture method. The killing-curves method was also performed against five strains with different susceptibility to azithromycin. The MIC(50) and MIC(90) of azithromycin were 32 and 64 mg/l, respectively. Moderate bactericidal activity was observed in 14 out of the 15 strains evaluated by the subculture method (MBCs from 1 to 4 dilution steps higher than the MICs) and by the killing-curve method. For three strains the number of CFU/ml was reduced 1 to 1.4 log by concentrations of azithromycin equivalent to 1 and 4 times their MICs. lt is concluded that azithromycin has moderate bactericidal activity against the strains of A. baumannii evaluated.