Span-length strings of letters, digits, or categorized words were presented for immediate recall, and, except for control strings, were prefixed by a single irrelevant and redundant element. The conceptual similarity of the prefix to the memory items was the independent variable of interest. It was found that letter prefixes were most disruptive of the recall of letter strings, as were digit prefixes of digit strings, and same-category word prefixes of word strings. Encoding of the irrelevant stimulus prefix appears to be obligatory; both semantic and nonsemantic features of the prefix can contact memory.