The presence of acinetobacters in a respiratory care unit was prospectively studied because of an increase in the number of isolations of Acinetobacter calcoaceticus. Cell envelope protein electrophoresis was used to distinguish strains. Eleven protein patterns were observed in isolates from patients and their environment. One pattern (pattern 1) was seen in several patients and environmental samples. Another pattern (pattern 2) was identified repeatedly in samples from skin and mucous membranes of patients in the same ward. After thorough cleaning was undertaken throughout the unit, the pattern 1 strain was no longer cultivated from clinical samples. It is concluded that cell envelope protein electrophoresis is a useful method for tracing epidemic strains of A calcoaceticus.