Biopsies from 89 patients with cervical carcinoma were studied using a clonogenic assay to obtain values for the surviving fraction at 2 Gy (SF 2). Heterogeneity in intrinsic radiosensitivity was investigated by independently processing multiple biopsies from 18 tumors. No significant differences between intra-tumour SF 2 values were demonstrated ( p = 0.30). The results have shown that intra-tumour heterogeneity is not a limitation to radiosensitivity testing using the Courtenay-Mills assay. A wide range of values (0.13–0.97) for SF 2 was obtained with a mean value of 0.47 ± 0.18 (±1 S.D., CV = 38%) for 52 squamous cell carcinomas and 0.59 ± 0.27 for four adenocarcinomas. There were statistically significant differences between the individual tumours ( p < 0.001). From the analysis-of-variance of all the SF 2 results it appears to be the surviving fractions below about 0.40 and those above about 0.7 which show significant differences in radiosensitivity between pairs of tumours ( p = 0.05). Also 36% of the values of SF 2 show significant differences from the mean SF 2 of all tumours. The storage of tumour cell suspensions in liquid nitrogen improved the colony-forming efficiency (CFE) but it did not alter the radiosensitivity.