The deformability of erythrocytes, which is a factor of utmost importance both for capillary perfusion and for determining erythrocyte life span, has been studied in patients with solid tumours and in healthy subjects. Deformability was significantly decreased in all patients, especially those with anaemia. In this latter group of patients, carboxyhaemoglobin saturation of blood (COHb), reflecting erythrocyte haemoglobin breakdown, was also significantly increased, and this increase was closely correlated with the degree of impaired deformability of the erythrocytes. In the group of patients without anaemia, where erythrocyte deformability was also impaired, no such correlation was found. In 4 patients with Hodgkin's disease and fever as part of B symptoms, erythrocyte deformability decreased during fever and returned to normal when fever subsided. In 2 of these patients the administration of a prostaglandin-synthesis inhibitor (indomethacin) evoked a deformability, which was even better than during periods without fever. Since fever in Hodgkin patients with B symptoms has been attributed to the production of prostaglandins, these results may suggest a relationship between prostaglandin production and erythrocyte deformability in patients with malignant tumours. The close correlation between the degree of decrease in erythrocyte deformability and COHb levels in the patients with anaemia might indicate a role for prostaglandins in the production of anaemia of malignancy.