Two experiments used an elicited speech-production paradigm to explore children's acquisition of noun case-marking inflections. Russian (N = 24, 2;10— 4;6 years) and Serbian children (N = 24, 2;10—4;11) were asked to produce prepositional phrases requiring genitive or dative inflections of masculine and feminine, familiar and novel, simplex (vaza [Ru/Se: vase]) and diminutive (Ru: vazochka, Se: vazica) nouns. Across languages, children produced fewer case-marking errors with familiar compared to novel nouns, and diminutive compared to simplex nouns. The diminutive advantage occurred despite a markedly lower frequency of diminutive usage in Serbian than Russian child-directed speech. This suggests that in acquiring richly inflected languages, children most readily construct low-level generalizations of inflectional changes applying to morpho-phonologically homogeneous clusters of words like diminutives.