Preliminary interpretive zone diameter criteria were calculated for the 5-micrograms cefdinir disk diffusion test by using two potential MIC breakpoints (less than or equal to 0.5 and less than or equal to 1 micrograms/ml). The absolute agreement between tests ranged from 85.9 to 92.4%, and the false-susceptibility errors were principally contributed by the Enterobacter spp. (2.2% error). One proposed criterion was greater than or equal to 20-mm zone diameter (less than or equal to 1 micrograms/ml) for susceptibility and less than or equal to 16-mm zone diameter (greater than 2 micrograms/ml) for resistance to cefdinir. Clinical laboratory users of the disk diffusion method should be cautioned about the possibility of very major interpretive errors among enterobacter isolates.