Mothers' provision of names for novel and familiar toy animals was examined during play interactions with 20 infants observed at ages 0;10, 1;1, 1;5, and 1;9. Of particular interest were characteristics of mothers' speech which might bear on children's development of lexical principles or constraints. Analyses demonstrated that mothers facilitated their children's determination of reference and differentially adjusted their naming practices to novel, comprehended, and familiar animals. They virtually always named the whole object first. More important, the first mention of novel, but not comprehended or familiar animals involved both maternal naming and physical designation of the object 92% or more of the time. Thus, although a novel word's referent may be indeterminate logically, mothers specify it practically. These results support the position that maternal labelling practices may assist children in acquiring lexical principles and that lexical acquisition, perhaps even the vocabulary spurt, can proceed during natural conversational interactions before infants master lexical principles.