Falsified and substandard medicines present serious concerns for public health. We describe an event that occurred in late 2011 involving falsified antiretroviral medicines found in the supplies of Médecins Sans Frontières (MSF) projects in Kenya. We discuss factors contributing to these falsified medicines entering the supply chain as well as the response by MSF and others. We make recommendations to help defend against future episodes of entry of falsified medicines into the supply chain as well as comments on appropriate responses in cases of falsified medicines.