Access to one's newborn infant is a commodity that can be traded for other benefits such as grooming in nonhuman primates. According to the biological market paradigm, the price paid should fluctuate with the number of newborns in the group. We investigated the grooming sessions between mothers with infants less than 3 months old and other adult females in free-ranging primates: one group of sooty mangabeys, Cercocebus atys, with 35 adult females (Ivory Coast) and two groups of vervet monkeys, Chlorocebus aethiops, with four and seven females, respectively (South Africa). Although many more infants were born per birth season in the mangabey group than in the vervet groups, interaction patterns involving infants showed many similarities: mothers did not reciprocate grooming received from non-mothers, but exchanged it directly for the opportunity to handle their infants, whereby obtaining access to infants required longer grooming bouts than reciprocating grooming in grooming sessions not involving infants. Low-ranking handlers needed to groom mothers for longer than their higher-ranking counterparts. The 'value' of an infant, in terms of grooming time received by the mother, decreased when infants grew older or when many infants were simultaneously present in the group. In vervets, infant availability affected handling times: females handled infants for longer when there were fewer infants. Furthermore, only frequent grooming partners of the mother could handle infants for longer and this familiarity was not kin related. This suggests that if the value of an infant varies with dominance, infant handling time may be determined by the quality of the females' relationships.