A fundamental step in learning words is the development of an association between a sound pattern and an element in the environment. Here we explore the nature of this associative ability in 12-month-olds, examining whether it is constrained to privilege particular word forms over others. Forty-eight infants were presented with sets of novel English content-like word-object pairings (e.g. "fep") or novel English function-like word-object (e.g. "iv") pairings until they habituated. Results indicated that infants associated novel content-like words, but not the novel function-like words, with novel objects. These results demonstrate that the mechanism with which basic word-object associations are formed is remarkably sophisticated by the onset of productive language. That is, mere associative pairings are not sufficient to form mappings. Rather the system requires well-formed noun-like words to co-occur with objects in order for the linkages to arise. (Contains 2 tables and 2 figures.)