We investigated the relationship between the acquisition of singular-plural morpho-syntax and children's representation of the distinction between singular and plural sets. Experiment 1 tested 18-month-olds using the manual-search paradigm and found that, like 14-month-olds (Feigenson & Carey, 2005), they distinguished three objects from one but not four objects from one. Thus, they failed to represent four objects as "plural" or "more than one". Experiment 2 found that children continued to fail at the 1 vs. 4 manual-search task at 20 months of age, even when told, via explicit morpho-syntactic singular-plural cues, that one or many balls are being hidden. However, 22- and 24-month-olds succeeded both with and without verbal cues. Parental report data indicated that most 22- and 24-month-olds, but few 20-month-olds, had begun producing plural nouns in their speech. Also, the success among the older children was due to those children who had reportedly begun producing plural nouns. We discuss a possible role for language acquisition in children's deployment of set-based quantification and the distinction between singular and plural sets.