Chronic anaemia, hyperbaric oxygen and tumour radiosensitivity. Anaemia is an important factor in the response of some human tumours to radiotherapy. The outcome is also influenced by whether the treatment is given in air or high pressure oxygen (HPO). The present study examined the relationship between anaemia and tumour response to radiation given in air or HPO in C3H mice transplanted with a mammary adenocarcinoma using a growth delay assay to assess the radiation response. Chronic anaemia was induced by the use of a low iron diet and was characterized by a significant reduction in host haematocrit and whole blood viscosity. In addition, anaemia was associated with a right shift in the oxyhaemoglobin dissociation curve and an increase in the volume doubling time of the tumour; but there was no change in the concentration of 2,3-diphosphoglycerate in the red cells. Radiation studies with these anaemic mice demonstrated that the tumour radiosensitivity was decreased when treatment was given in air. HPO was successful in overcoming the increased radioresistance associated with anaemia. This result suggested that tumours grown in anaemic mice have a higher hypoxic fraction than those grown in control mice. Changes in host physiology with chronic anaemia may contribute to the benefit seen with HPO but such alterations per se may be inadequate to maintain tumour oxygenation when treatment is given in air. 