The usefulness of sodium dodecyl sulphate–polyacrylamide gel electrophoresis patterns of cell envelope proteins for classifying strains of Acinetobacter calcoaceticus was studied using 129 isolates from 16 in-patients in a teaching hospital. In 11 patients, all of the isolates from each patient exhibited the same pattern irrespective of the body site or time of isolation. The patterns of the isolates from four other patients were indistinguishable, with the exception of one isolate per patient. In the isolates from one patient five patterns were observed. In several cases isolates from different patients exhibited the same pattern. The relative frequency of some of these patterns was low. Epidemiological data were compatible with the assumption that the concurrent presence of bacteria of these patterns in the patients was the result of cross-infection. For one pattern, which was seen in seven patients, cross-infection could not be substantiated. On the basis of analysis of electrophoretic patterns in combination with epidemiological data on a number of strains it is concluded that cell-envelope protein profiles appear to be a useful aid in studying the dissemination of Acinetobacter in the hospital environment.