The diagnosis of HIV infection in children born to HIV-1 infected mothers is hampered by the passive transfer of maternal IgG across the placenta and remain detectable up to 12 or even 15 months of age. We have therefore employed a simple, cheap, sensitive and specific test, the Antibody class A specific capture enzyme immuno assay (AAC-EIA) and class M specificapture enzyme immunoassay (MAC-EIA) for the detection of child's own antibodies, the IgA and IgM anti HIV-1. Contrary to the currently held dogma, I demonstrated that the IgA and IgM do cross the placental barrier. The maternally derived IgA and IgM clear and are undetectable by 3 months, but from this time, detection of IgA and IgM in infant samples is a strong indicator of HIV infection. Detection of postnatal transmission of HIV-1 through breast milk is also discussed. A highly sensitive and specific method for serotyping HIV-1 was developed and applied. The method provides a way of typing HIV-1 by a serological method. It also provides a way to measure the level of maternal antibodies in infecting and non-infecting mothers. 2) The sequencing of proviral DNA or plasma reverse transcribed RNA remains the method of choice for subtyping of HIV-1. The method was used in this study. A remarkable molecular heterogeneity was observed in a small number of patients examined. In Kimpese, at least 6 subtypes (A, C, D, F, G, H) and another possible new subtype of HIV-1 are co-circulating. This study is the first to have recruited and followed up a cohort of mothers and children from this rural part of Africa (DRC, ex Zaire). It provides a practical approach to early diagnosis of HIV-1 infection in children and the method can be transferred to laboratory services in developing countries. In addition, a simpler method of qualitative and quantitative analysis of antibodies against the principal neutralising domain of HIV-1 V3 loop and the subtyping of vertically transmitted HIV-1 isolates from Kimpese has been described.