The present study aims at examining the relative importance of various acoustic features as cues to familiar speaker identification. The study also attempts to examine the validity of the prototype model, as the key to human speaker recognition. To this aim 20 speakers were recorded. Their voices were modified using an analysis-synthesis system, which enabled analysis and modification of the glottal waveform, of the pitch, and of the formants. A group of 30 listeners had to identify the speakers in an open-set experiment. The results suggest that on average, the contribution of the vocal tract features is more important than that of the glottal source features. Examination of individual speakers reveals that changes of identical features affect differently the identification of various speakers. This finding suggests that for each speaker a different group of acoustic features serves as cue to the vocal identity, and along with other predictions that were found to be valid, supports the adequacy of the prototype model.