Children with deficits in language comprehension have been shown to be at the greater risk for persistent clinically significant language delay. There is a low availability of standardized norm-referenced instruments for assessing early receptive abilities. This study shows data collected on 196 children (18-48 months) with a new acting-out task for the assessment of early grammar comprehension. The progression of task's items is based on developmental emergentist theories. Results show that the task is able to describe the development of grammar comprehension between 18/24 and 48 months. At 18 months children are not able to process grammar to perform the actions requested. Until 30 months children tend to process grammar relying on semantic cues. At 36 months there is evidence of stable use of word order as syntactic cue and only from 42 months children show to master more complex grammatical cues to understand passive sentences and sentences with pronouns. The described task could be useful to identify children with developmental difficulties and to delineate their profiles for planning specific interventions.