Twenty-five patients treated for malignant conditions of the head and neck were investigated for the development of oral radiation mucositis as well as oropharyngeal colonisation with fungi and Gram-negative bacilli. The colonisation rate of the patients increased from 20% to 80% during therapy. The patients were colonised mainly with fungi, but also to a lesser extent with Gram-negative bacilli. During radiotherapy free volatile fatty acids in saliva were determined by direct gas-liquid chromatography. The amounts of acetic, butyric and propionic acid decreased during radiotherapy and remained at low levels 6 months after therapy. Radiation induced mucositis was observed in 28% of the patients after 1 week of irradiation. After 3 weeks of therapy all patients showed varying grades of mucositis. In seven of the 19 survivors (36%) erythema was still present 6 months after radiotherapy.