This trial will be conducted in Zimbabwe and Malawi. The Phase III trial is preceded by a 2-week safety and acceptability pilot study to determine the extent of genital irritation caused by N-9 and the placebo. Following the pilot study and prior to screening for enrollment in the Phase III trial, potential volunteers will participate in a 2-month behavioral counseling program that will encourage the use of condoms. Only participants who decide they cannot or will not be consistent condom users are enrolled in the main study. Study participants are given N-9 gel or placebo to use each time they have vaginal sexual intercourse for 18 to 36 months. Participants have a follow-up evaluation once a month; a pelvic exam and blood tests are performed every third month.