A series of tests were designed to assess differences in behavioural characteristics between immature rhesus monkeys. The tests mostly involved responses to strange objects or behaviour in mildly disturbing or frustrating situations. They were administered when the infants were 6, 12, 18 and 30 months of age. Rank order correlations between the same measure in replicated tests, between different measures in one test, between measures from different tests, and between measures from the tests and data from routine observations on mother-infant interaction and on infant activity, were calculated: these were used to assess the reliability and relations between the measures used. The tests were used to assess differences between groups of monkeys differing in previous experience such as presence or not of companions other than the mother, or having been separated from their mother or not. Despite the small group sizes, some significant differences appeared.