What is the source of interference on a memory test following study of a list containing different types of pairs? Many current models predict that pairs and singles of all types will jointly interfere and therefore harm memory. Such list length effects have often been observed for lists of a single-item type (e.g., a list of words). Here, we examine interference for lists containing multiple types of pairs (e.g., word-word, face-face, word-face). In three experiments, we manipulate the number of each type on the study list. In associative recognition, discrimination fell as the number of pairs of the same type rose, but the number of pairs of other types had little effect. That is, we found a list length effect within, but not between, classes of stimuli. We highlight the importance of representation and propose alternatives to current model representations that can predict such findings.