Neutralization/enhancement of HIV-1BaL by maternal and infant plasma diluted 10-1 – 10-3. Virus replication (determined by p24 antigen (pg/ml) in culture fluids collected 2–4 days post-infection) is compared to replication of virus treated with plasma from HIV-1 seronegative (sn) women. The data represent the average of all maternal and infant samples. Two independent infections were conducted with each sample in duplicate (4 replicates) for each of 43 maternal and 45 infant plasma. Maternal and infant plasma significantly reduced HIV replication (p = 0.001 for both) at 10-1 dilution compared to HIV negative sera. Infant plasma also significantly reduced HIV replication (p = 0.001) at 10-2 dilution. At 10-3 dilution maternal plasma significantly increased HIV replication (p = 0.005) above HIV negative sera and infant plasma showed a non-significant increase (88%) in HIV replication.