In this study we systematically investigate the mode of acquisition and the developmental process of stone handling, a form of solitary object play, in a captive troop of Japanese macaques (Macaca fuscata) housed in an outdoor enclosure at the Kyoto University Primate Research Institute, Japan. This study was conducted to evaluate two alternative hypotheses regarding the mode of acquisition of stone handling in infants: (1) environmental stimuli (availability of and exposure to stones) and (2) social stimuli (exposure to stone handling individuals). Early exposure to stones in the environment had no significant effect on when infants acquired the behavior. No significant correlations were recognized between the age of stone handling acquisition and number of stones encountered per hour from birth to acquisition, or the time spent in a specific area of the enclosure as a function of the number of available stones therein. However, being exposed to a stone handling model(s) was a social stimulus that had an effect on the age of acquisition, with a significant negative correlation between a mother's stone handling frequency and the age of acquisition by her infant. Infants of non-stone handling mothers acquired the behavior much later than others. Infant peers who acquired stone handling earlier played no significant role as stone handling models. Of the factors tested here, the timing of acquisition depended mainly on the level of proximity to a demonstrator and the frequency at which those available demonstrators performed the behavior.