The transient evoked otoacoustic emission (TEOAE), generated by a click and recorded in the ear canal is influenced by many factors. Of these, the form of the stimulus, the characteristics of the microphone, amplifiers and filter are probably the major ones. In addition, the acoustical loading of the ear canal by the probe may interact with the TEOAE measurement. Data presented here were obtained from 64 babies tested on each of 3 days post partum. The differences in the TEOAEs produced by the ILO88 and POEMS instruments can be quite marked. The ILO88 TEOAEs consistently have larger high frequency components than do the emissions from the POEMS system. The ILO88 system also gives higher correlations between repeat recordings. Such differences point out the need for explicit standards in probe design.