New antiretroviral agents or combinations are sought that are as efficacious as ZDV and that would be effective in reducing the rate of vertical transmission of HIV in women who have been long-term recipients of ZDV. d4T is a good candidate drug. It is a thymidine nucleoside analogue that inhibits replication of HIV at concentrations similar to those of ZDV which have anti-HIV activity. The demonstrated safety profile of d4T, the ease of administration, and, most importantly, preliminary efficacy data, especially in combination with 3TC, make this an excellent candidate combination for a Phase I perinatal trial.