Vaginal microbicides are heralded as a woman's HIV prevention method. This study, conducted in a microbicide clinical trial setting in Zambia, explored how the social construction of masculinity and sexual behaviour influenced the acceptability of vaginal microbicides. The data were generated from 18 In-depth Interviews and 8 Focus Group Discussions. The data were analysed thematically. The study found that hegemonic masculinity influenced the use of vaginal microbicides positively and negatively, in multiple ways including: decision to initiate gel use, autonomous use of the gel, and consistent use of the gel. Men were seen as heads of households and decision-makers who approved their partners' intentions to initiate gel use. Autonomous gel use by women was not supported because it challenged men's dominant position in sexual matters and at a family level. The socially accepted notion that men engaged in multiple sexual relationships also influenced women's decision to use the gel. Sustained gel use depended on the perceived effect of the gel on men's sexual desires, sexual performance, fertility, and sexual behaviour. This study suggests that acceptability of microbicides partially lies within the realm of men, with use constrained and dictated by cultural constructs and practice of masculinity and gender.