Infants aged 4 and 6 months were presented with events in which a person acted so as to set another person, or an inanimate object, in motion. In one condition, the actor spoke to the person (natural) or inanimate object (unnatural); in the other condition, the actor grasped and manipulated the person (unnatural) or object (natural). Six-month-old infants looked reliably longer at the natural actions than at the unnatural actions. A follow-up experiment revealed that their preference depended on the naturalness of the human actions themselves, not on the features or motions of the person or object that was acted upon. Looking preferences at 4 months were equivocal, consistent with the thesis that sensitivity to the natural actions develops over the first 6 months of age. We discuss these findings in relation to the development of social understanding, social gaze, and visual exploration. (Contains 6 figures and 4 footnotes.)