The present method for predicting the radiocurability of individual human tumors is based upon considerations of tumor size, site, histological type and grade, and host factors such as sex and age. Small tumors located such that normal tissues do not seriously limit total dose and those with "favorable" histology are more radiocurable than large tumors located over a critical normal tissue. However, the precision of prognosis based upon those features is relatively low. The need for other parameters for more accurate predictability is greater than ever because of the existence of different radiation modalities, including neutrons, and the development of a broad range of chemotherapeutic drugs that can be used alone or in combination with radiation and surgery. In this laboratory we are testing the micronucleus (MN) assay for measuring the relative biological effectiveness of high- and low-LET irradiations in mouse tumors and a new primary human tumor cell culture system for making direct measurements of tumor cell radiosensitivity. The potential usefulness of these two systems in predicting human tumor response to radiotherapy is discussed.