There is a critical need to develop new strategies to prevent sexual transmission of HIV. Condoms have limited acceptance, and a vaccine may not be available for many years. A vaginal formulation could provide an alternative method if a compound that inhibits sexual transmission of HIV can be identified or synthesized, and if this agent can be formulated for vaginal use. In this report we describe an infection assay for testing compounds that may be useful in a vaginal formulation. This assay system utilizes a cell line (ME-180) derived from the human cervix which, on the basis of morphological features, is an appropriate model of female and male genital and urinary tract epithelia. These cells can be productively infected with HIV upon exposure to HIV-infected T-cell lines. Blocking experiments can be readily carried out in this model because in this p24 ELISA assay system the quantity of virus released by the infected epithelium over a 24-h period is 40 times background.