Irradiation of the salivary glands produces a rapid increase of salivary amylase in serum, released by the highly radiation sensitive serous cells of the glands. Serial assays of salivary amylase in serum were performed in patients treated by radiation to the upper neck region. The changes observed were compared with the amount of salivary gland mass irradiated and with the dose fractionation modality used. The irradiated volume included either the entire salivary gland mass or less than 50 per cent of the gland. Two fractionation modalities were used: a conventional fractionation of 2 Gy per day, 5 times a week, or a multiple daily fractionation of 2 Gy, 3 times a day in two series of 4 days with a 4-day interval. Both parameters (salivary gland mass irradiated and fractionation modality used) significantly influenced the shape of the amylase curve in the serum. Serum amylase may therefore be considered a reliable biologic indicator of early injury to the salivary glands.