pH and salivary lactobacilli were analysed in 22 healthy individuals and 34 patients with cancer tumors of the oral or oropharyngeal regions before and during irradiation therapy. The findings revealed in cancer patients before radiotherapy, that pH and lactobacilli were significantly lower than those in non cancer control subjects. During irradiation therapy, pH and lactobacilli were not significantly altered. No significant difference was found between patients cured by chemotherapy before radiotherapy and patients treated only by irradiations. However, pH is modified by teeth: toothless patients ph and toothed patients pH change in opposite direction.