Where does the mutual exclusivity bias to map novel labels onto novel objects come from? In an intermodal preferential looking task, we found that novel labels enhance 10-month-olds interest in a novel object over a familiar object. In contrast, familiar labels and a neutral phrase gradually reduced attention to a novel object. Markman (1989, 1990) has argued that the name for a familiar object has to be recalled to rule out the object as the referent of a novel label. Yet, at 10 months of age, infants attention might be guided by the novelty of objects and labels, rather than knowledge of the names for familiar objects. Mutual exclusivity, as a language-specific bias, might emerge from a more general constraint on attention and learning.