Every aspect of the life of the captive non-human primates should be carefully attended to, as updated refinement concept recommends. Interaction with humans as environmental enrichment for these animals is believed to be of value, but it has been subject to little quantitative evaluation. This study investigates the effects of positive interaction with humans on the behaviour of a captive colony of common marmosets (Callithrix jacchus). The study was composed of two phases: baseline condition, where the interaction with humans was represented by routine care and management; and a second phase (‘Human Interaction Effects’), in which a familiar caretaker spent additional 20min per day with each family, interacting actively and positively with the monkeys. In order to assess potential durable effects of such interaction, data were collected only when caretaker was absent. Between the two phases, a period of interaction 4 weeks long per family took place with the caretaker. The sampling method used was a 10s scan sampling, with daily sessions 30min long. Following the period of additional interaction with caretaker, the marmosets showed an increased level of grooming and playful activities, generally considered signs of increased level of welfare; however, they also showed lower levels of self-scratching and locomotion. A trend towards reduced contact vocalizations was also observed. These results suggest that simple, unstructured positive interactions between humans and marmoset monkeys may be part of a program aimed at maximising the level of welfare of captive non-human primates.