MICRO-ID (General Diagnostics, Morris Plains, N.J.) is a new kit system designed for the identification of Enterobacteriaceae in 4 h. It consists of 15 biochemical tests of paper disks. Each test is in its own compartment in a molded plastic tray. Only one reagent need be added to the system (2 drops of 20% KOH, which is added to the Voges-Proskauer test). Based on the pattern of positive and negative biochemical test results, a five-digit octal code number is calculated. An identification is derived from a computer-generated identification manual. A study was conducted to compare three systems-the MICRO-ID 4-h and the API 20E (Analytab Products Inc., Plainview, N.Y.) 18- to 24-h systems and a conventional media system-to measure the ability of each to identify members of the family Enterobacteriaceae. Comparison tables, rather than simple percentage agreement tables, were generated to define the particular strengths and weaknesses of each system and allow the laboratory to best use the data. The MICRO-ID compared quite favorably with conventional media. MICRO-ID yielded incorrect identifications with 1.5% of the isolates tested (API 20E, 4.7% misidentification rate). Half the MICRO-ID misidentifications occurred when the system identified a Citrobacter diversus as a lysine-negative Escherichia coli; all gave one octal number. A direct comparison of the MICRO-ID and API 20E was of limited value because percentage agreements were merely the sums of the errors of each. The ease of inoculation, the requirement for the addition of only one reagent, and the 4-h capability make the MICRO-ID system an extremely attractive development in the field of bacterial identification.