Twenty-month-old children learned to recognize nonsense labels for five novel object concepts and were tested on generalization to variants of these concepts. Children were presented with either one or three examples of each object type during learning sessions. Results showed that receptive learning of names for object concepts was significantly related to a number of possible manipulations specific to each object type and to labelling by children. Children's generalization choices were consistent with adults' ranking of similarity of variants to concept prototypes. Children who learned less well were more likely to generalize to new instances of an object concept and to a greater number of variants if they had been exposed to three rather than one example during training sessions. Results also support the hypothesis that differentiation of objects in interaction is important to the formation of an object concept at this age.