Institutional barriers can hinder effective access and utilisation of Newcastle disease vaccines among smallholder chicken farmers. Many studies have focused on formal institutional barriers with minimal focus on informal institutions - unwritten rules and regulations that govern access and utilisation of Newcastle vaccines. However, informal institutions are more profound and encultured in individuals’ daily activities. This study sought to investigate informal institutional barriers to access and utilisation of Newcastle disease vaccines among women smallholder chicken farmers in Makueni, Kenya. The cross-sectional qualitative study employed in-depth interviews, key informant interviews and focus group discussions as data collection methods. Study informants were conveniently and purposively sampled. Informal institutional barriers to access and utilisation included: fear of Newcastle disease vaccine as a new technology, use of herbal remedies, mistrust of community vaccinators, gender division of labour, ownership of household resources and beliefs that indigenous chickens do not need vaccines. The study concludes that women chicken farmers are constrained by unwritten rules, norms, regulations and gender roles that hinder their access to and utilisation of the Newcastle disease vaccines. The need to examine informal institutions to identify and eradicate barriers to access and utilisation of Newcastle disease vaccines by farmers is recommended.