The Vitek AMS automated instrument method for identification of Enterobacteriaceae was compared with two rapid manual methods intended for the same purpose, the Micro ID System and the API 20E Same-Day procedure, on a series of 400 consecutive fresh clinical isolates. Results were compared with identifications obtained using the API 20E System with overnight incubation and supplemental tube biochemicals (when needed). Both the final (8-hour) and a manually requested, presumptive 5-hour result from the AMS were compared with the 4-hour results provided by the Micro ID and the 5-hour results provided by the API. The Micro ID system proved to be the most rapid and accurate of the three test systems by correctly identifying 96.8% (387/400) of isolates. The API 20E using 5-hour readings identified 90.7% (363/400) of isolates, although 96.8% (387/400) could be identified if supplemental overnight tests were employed to separate profile codes with "good likelihood, but low selectivity." The AMS correctly identified 88.8% (355/400) isolates after 5 hours, and 95.0% (380/400) following 8 hours incubation.