During January, 1976 seven patients in an SICU became colonized or infected with Acinetobacter calcoaceticus (variation, anitratum) at an attack rate of 12.5 per cent. The organism showed a marked reduction in antimicrobial sensitivity from previous experience. Comparison with 34 uninfected control patients indicated that intubation and continuous ventilatory assistance were significantly associated with acquiring the organism (p = 0.0154). Acinetobacter was cultured from nurses' hands, AMBU adaptors, respirator apparatus, respirometers, and unlabeled bottles of saline used for tracheal irrigation. Control measures were designed to interrupt transmission. The investigation identified the nursing techniques and reservoirs that allowed this outbreak to occur, and we emphasize the need for close surveillance of patient care procedures in an intensive care unit.