The relationship between in vitro radiation sensitivity and colony-forming efficiency has been examined for primary human tumors of the cervix, colorectum, and lymph gland. Tumors were cultured using the Courtenay-Mills soft agar clonogenic assay and radiosensitivity was assessed as surviving fraction at 2 Gy. There was no correlation between colony-forming efficiency and surviving fraction at 2 Gy, giving confidence to the use of surviving fraction at 2 Gy as a predictor of clinical outcome.