Word-fragment completion is a frequently used test in implicit memory research. A database of 196 Spanish fragments was recently published (Dasí, Soler, & Ruiz, 2004) in which the fragments were described for indices, such as difficulty, familiarity, frequency, number of meanings, and so on (www.psychonomic.org/archive). In this work, a new index, thepriming index, is described for the same 196 fragments. This index is calculated for each fragment by subtracting the difficulty index (the proportion of correct completion when the fragment is not studied) from the proportion of correct completion when the fragment is studied, and it means the capacity of an item to be primed. In order to determine whether the new index performs well, we have replicated some important experimental effects, such as priming, frequency, and difficulty. We consider that this index can help researchers to better select stimuli for fragment-completion tasks.