Polymerase chain reaction (PCR), virus culture (V), antigen detection (Ag), andin vitroantibody production (IVAP) assays may be useful for the early detection of vertically transmitted HIV-1 infection in infants under 18 months of age, when a diagnosis cannot be based on seropositivity because of maternal antibody persistence. To assess the reliability of these procedures and to correlate diagnostic results with infection status, 101 children born to HIV-1-seropositive mothers were evaluated by all these techniques within the first 6 months of life. The children were then followed up to the age of at least 18 months, when diagnosis was made on the basis of AIDS or AIDS-related complex (ARC) onset or persistence of HIV-1 seropositivity. Out of 27 children classified as infected according to the above criteria, 25 (92.5%) were repeatedly positive in IVAP test, 22 (81.5%) in the first PCR analysis, and only 19 (70.3%) in the initial V assay. On further testing, a total of 24 children (88.9%) were found positive in PCR assay, and 23 (85.2%) in V test. All these assays were found to be more sensitive than antigen detection for HIV-1 infection diagnosis, but the antigenaemia was shown to be a useful prognostic marker of disease onset. We also found that both Ag and IVAP assays could give false-positive results in the first 2 months of life, which severely limits their diagnostic value during this period of time. False-positive results in PCR assay could occur at any time of the tested period and were unrelated to the child's age. Interestingly, positive results in both PCR and V assays were obtained in three out of 74 asymptomatic children who lost HIV-1 antibodies.