Dried blood spot samples from newborn babies have been successfully tested for HIV-1 antibody by the particle agglutination method to assess the prevalence of infection in the mothers. In January, 1990, unlinked anonymous testing of Guthrie cards for HIV antibody was begun in Scotland. 99.6% of Scottish births were tested. 9 mothers spontaneously refused to allow testing of their baby's blood. Samples were coded by district postcodes. Eluates of 65,773 dried blood spots were initially tested for HIV-1 antibody with the Fujirebio technique. Of the 31 positive samples 19 were confirmed to be positive by enzyme-linked radioimmunoassay and western blot (seroprevalence 0.29 per 1000). All these samples came from large metropolitan areas on the east coast. Prevalences were 2.5 per 1000 for Edinburgh city, 1.4 per 1000 for Dundee, and 0.7 per 1000 for Aberdeen. We identified as HIV-positive all babies known to be so in named testing programmes. HIV testing of Guthrie cards can be used to monitor HIV status in mothers who have just given birth. The use of district postcode data in sample identification will allow accurate targetting of prevention strategies and early detection of spread of infection by geographic area.