We studied the overall acceptability of UC781 gel formulation when applied rectally. Ten women and twenty-six men, all HIV-uninfected, were enrolled in a Phase 1, randomized, blinded, placebo-controlled safety and acceptability study of the vaginal microbicide gel UC781 applied rectally. Participants were randomized to three groups: 0.1% UC781 gel, 0.25% UC781 gel, or a placebo gel. Acceptability was assessed using structured questionnaires and qualitative in-depth interviews. After using UC781 gel rectally for seven consecutive days, participants' reports suggest that a UC781 gel formulation is highly acceptable and comparable to a placebo gel. The gels received favorable ratings overall and on attributes such as color, smell and consistency. All of the participants reported high intentions to use a gel like the one they used in this study. Acceptability research is essential in early phases of microbicide development to identify potential problems, understand user preferences, and introduce changes if needed.