This study of men who have sex with men (MSM) examined preferences about the characteristics of a potential product for preventing sexual transmission of HIV, such as a rectal microbicide. MSM were recruited in West Hollywood, California. They self-administered a questionnaire and rated 48 product characteristics representing seven dimensions. Overall, the ratings were highest for effectiveness in preventing HIV and other sexually transmitted diseases, followed by characteristics reflecting the physical or secondary effects of the product and logistics of use. Physical attributes, convenience/accessibility, and psychological aspects had intermediate ratings; interpersonal dynamics had the lowest rating. Men with negative attitudes about using condoms to prevent HIV infection were more likely than their counterparts to prefer a product that does not reduce sexual sensation or pleasure, does not break the mood, and can be used after a sexual encounter ends. A similar pattern was observed when participants were stratified by whether or not they had engaged in unprotected anal intercourse in the past 12 months. The findings inform the development, testing, and marketing of a future HIV prevention product for MSM.