A capture enzyme-linked immunosorbent assay (ELISA) to distinguish between blood from children and adults in the mosquito blood meal was examined using the α1 chain of the aminopropeptide of human procollagen type I (PINP) as antigenic marker. Rabbit anti-human PINP (α1) antibody was used as catching antibody, and either normal serum from 288 African and 58 Caucasian children and adults, or blood meals from 93 fed Aedes aegypti, were examined. PINP in excess of 40 optical density units (ODU) was detected in all children aged 0–13 years, whereas adults aged 21–77 years had PINP levels less than 25 ODU. In the transitional age group (14–20 years), the PINP levels ranged from 1 to 166 ODU. The PINP levels in 95% of the mosquito blood meals obtained from children exceeded the control levels, using 13 ODU as a cut-off value, whereas none of the blood meals from adults exceeded 13 ODU. The PINP levels in the mosquito blood meals were constant 1 and 8 h after ingestion, but they had decreased significantly after 16–19 h. Our data suggest that the test can be used to identify host preferences in studies of mosquitoes collected within 16 h after the blood meal. A field evaluation is necessary to determine the potential of the antigenic marker PINP as a tool in the identification of mosquito host preference.