1. It has been suggested that the physiological consequences of strenuous exercise are analogous to those of the acute-phase response. 2. In 70 male and 20 female competitive distance runners, a marked, but transient, neutrophil leucocytosis occurred immediately after these athletes completed a standard (42 km) marathon race. Concomitant significant increases were noted in the plasma cortisol levels, creatine kinase activity, C-reactive protein level, total protein level and albumin level (P less than 0.01). 3. The plasma fibrinogen, C-reactive protein and total protein concentrations were markedly increased both 24 h and 48 h after exercise (P less than 0.01). The serum haptoglobin level was significantly decreased after exercise (P less than 0.01), and increased 48 h later (P less than 0.05). There was no change in the serum iron level, total iron-binding capacity, per cent saturation of transferrin and serum ferritin level. 4. A significant increase in interleukin-1-type activity was demonstrated immediately and 24 h after exercise (P less than 0.01). 5. It is concluded that the metabolic sequelae of sustained exercise are similar, but not analogous, to the acute-phase response, and interleukin-1, probably plays a significant role in linking the haematological and immunological changes observed after sustained strenuous exercise.