The objective of the study was to indicate HIV infection in infants. The patients were part of a longitudinal cohort of 43 infants born to HIV seropositive mothers. A modified Genelavia EIA primarily directed against HIV envelope proteins was used. An alkaline phosphatase labelled IgG3 conjugate was substituted in place of the kit conjugate. HIV specific IgG3 clearance was optimal at 6 months, whilst HIV total antibody was reliable only from age 12 months onwards. At 6 months no detectable IgG3 were found in 91 per cent of uninfected infants where more of these infants had reduced their total HIV antibody titres at the same period. We confirm that HIV specific IgG3 measurement is a reliable and cost effective means of identifying HIV infected infants from 6 months of age onwards.