Recently, attention has focused on the potential of oncogenes, tumour suppressor genes, and assessment of cell proliferation as biological response indicators in human cancer. In this study, immunocytochemical analysis was used to evaluate the usefulness of Ki67, epidermal growth factor receptor (EGFr), and the protein products of c-Myc and Bcl-2 as indicators of radiosensitivity in primary cultures of head and neck tumours. Primary cultures established from tumours taken at surgery were divided into two groups; the control group remained untreated, and the treatment group received a single dose of 2 Gy. The cultures were incubated for 14 days, after which time they were fixed and examined immunocytochemically. The response to treatment of the cultures was measured as the percentage of growth inhibition (%GI) in the treated cultures relative to the untreated controls. Expression of Ki67 measured after a single dose of 2 Gy significantly differentiated the radioresistant and radiosensitive groups (P = 0.045); high percentages of Ki67+ cells correlated with radioresistance. A significant difference was found between the expression of EGFr in the resistant and sensitive groups, as measured in control cultures and after a dose of 2 Gy (P = 0.002 and P = 0.03, respectively); high levels of expression of EGFr correlated with radioresistance. The level of expression c-Myc+ cells, as measured in control cultures, significantly distinguished the radiosensitive group from the radioresistant group (P = 0.05). These results indicate a potential role for these proteins as indicators of radioresistance.