Hereditary pulmonary alveolar proteinosis (herPAP) is a rare disease characterized by the slow build-up of lipo-protein material in the lungs due to the failure of highly specialized cells called macrophages, which usually eat away this material from the pulmonary air-space. Researchers demonstrate that a single transplantation of murine macrophages into the lungs of mice suffering from herPAP can treat this life-threatening disease.