To dissociate the influence of host age from length of exposure on acquisition of filarial infection, we examined the development of microfilaraemia and anti-filarial IgG4 in all ages of a naive population that became suddenly exposed to Brugia malayi as a result of transmigration. Responses in 247 transmigrants, who had settled for periods of several months up to 6 years in their new homesteads, were compared with those of 133 life-long residents. As shown in earlier studies, anti-filarial IgG4 increased with age in the indigenous population, whose age is equivalent to length of exposure. However, by examining transmigrants, it became clear that development of specific IgG4 was influenced by age, since levels of this antibody were consistently higher in transmigrant adults than in transmigrant children, despite an equal length of exposure to filarial infection. Examining microfilaraemia, it was confirmed that infection establishes more rapidly in adults than in children.