Semantic intrusions are inappropriate responses frequently observed in patients with Alzheimer's disease. They belong to the same category as the words to be remembered, but their prototypic value remains largely unexplored. The prototype is the most representative word in a particular lexical category. The prototypic value is measured according to different criteria: written and oral lexical frequency, frequency of use, degree of typicality, degree of familiarity and rank of quotation. The objective of the study was to evaluate the prototypic value of intrusions produced by 17 Alzheimer's patients with mild to severe dementia, during the cued recall of the Grober & Buschke procedure (RL/RI 16 items). The prototypic value was compared to the categorial norms provided by 1) 17 control subjects and 2) the lexical database "Lexique 3". The results show that intrusions had a significantly higher prototypic value than targeted items. The prototypic value increased with the progression of the disease, and according to the evaluation criteria used. Thus with the criteria "frequency of use", "degree of typicality" and "degree of familiarity," the prototypic value increased exponentially with the severity of dementia. In contrast, in spite of the development of the pathology, the prototypic value decreased when assessed by the criteria of "rank of quotation", and "lexical frequency" (oral and written). In conclusion, the qualitative analysis of the prototypic value of intrusion errors in Alzheimers opens up new clinical and methodological considerations.