What processes do infants employ in categorizing? Infants might categorize on line as they encounter category-related entities; alternatively, infants might depend on prior experience with entities in formulating categories. These alternatives were tested in forty-four 5-month-olds. Infants who were familiarized in the laboratory with a category of never-before-seen objects subsequently treated novel objects of the same category as familiar--they categorized on line--just as did infants who were exposed to objects from the same category at home for 2 months leading to their laboratory assessment of object categorization. Infants with home experience also recognized novel category objects as familiar from the outset--that is, prior experience with category exemplars was brought to bear in laboratory tasks.