African green monkeys (Cercopithecus aethiops) are naturally infected with simian immunodeficiency virus of African green monkeys (SIVagm)(Ohta et al., 1988) and characterization of the genome of this virus, demonstrated that it is a lentivirus related to HIV-1 and HIV-2 (Chakrabarti et al., 1987; Fukasawa et al., 1988), the aetiological agents of AIDS in humans. The SIV group of viruses in Africa represent the closest known relatives of HIV (Chakrabarti et al., 1987) and have a very similar pathobiology in the monkey host (Daniel et al., 1985). About 30% of the wildcaught African green monkeys tested are seropositive for SIV antibodies and about 4% of the sexually mature monkeys convert from seronegative to seropositive each year (unpubl. obs.).

We used SIV infections in six, pregnant female African green monkeys as a natural model of HIV, to determine the risk of vertical transmission. Blood samples were taken at intervals from these animals (before they were pregnant, at parturition and then bi-monthly for 1 year) and from their offspring (immediately after birth and then bi-monthly for 1 year). The sera from these samples were tested, in several independent laboratories, for antibodies to SIVagm, using ELISA and Western blots (Carlson et al., 1985), and peripheral blood mononuclear cells (PBMC) were cultured for the virus. Although the patterns of