A technique for typing Acinetobacter sp. by bacteriocin production has been developed. One hundred and seventy-six cultures from patients in outbreaks, in the community and environmental sources were identified, tested for sensitivity to gentamicin and bacteriocin typed; 154 were A. anitratus and the remainder A.lwoffi. Only one A.lwoffi strain produced bacteriocin. Ten of 22 were sensitive to bacteriocins and could be used as indicators. A close association was found between bacteriocin production and gentamicin resistance: MIC 4 mg/ml. Using six indicator strains 100/104 (96%) gentamicin-resistant strains were typed with nine distinct patterns of inhibition. Overall typability was 65% but 100/176 (56%) fell into only two groups. The technique may be of value in studying the epidemiology of Acinetobacter.