The outer membrane protein (OMP) profiles of 23 blood isolates of Acinetobacter baumannii representing all the different antimicrobial susceptibility patterns observed during a 3-year period in a Spanish hospital were studied. OMPs extracted from envelopes of sonicated cells after solubilisation with 2% of N-lauryl-sarcosinate were analysed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) using the Laemmli's buffers. Eight running gel systems differing in the concentration of polyacrylamide (8%, 10% and 12%) and in the absence or presence of urea (4 M and 6 M) were used in a preliminary study analysing the OMP profiles of four clonally unrelated strains of A. baumannii. When this study was completed, the OMPs of the 23 A. baumannii were analysed in 10% SDS-polyacrylamide gels with 6 M urea and in 12% SDS-polyacrylamide gels. Ten OMP profiles were observed in 10% SDS-polyacrylamide gels with 6 M urea, whereas only 5 OMP profiles were visualised using 12% SDS-polyacrylamide gels. The OMP profiles obtained in 10% SDS-polyacrylamide gels with 6 M urea only partially correlated with those observed in 12% SDS-polyacrylamide gels. In conclusion, the use of 10% SDS-polyacrylamide gels with 6 M urea is recommended for the study of OMP profiles of A. baumannii.