A simple and reproducible model of acute haematogenous staphylococcal osteomyelitis is described. Twenty-nine day-old chickens were inoculated intravenously with 10(4)-10(8) viable organisms Staphylococcus aureus per kg body weight and were killed 1-8 days after inoculation. Macroscopic septic foci could be detected within 24 hr of inoculation and were most commonly situated in the metaphyseal region of the proximal tibia and distal femur. Lesions in other organs were not observed. The production of osteomyelitis was dependent on the bacterial inoculum size. It was estimated that 5.5 X 10(5) viable organisms per kg body weight of chicken were required to produce osteomyelitis in 50 per cent of injected chickens. Chicken weights were monitored throughout the experiment. A close negative correlation existed between the logarithm of the bacterial inoculum size and the chicken growth rate in the first 24 hr following inoculation (r = -0.968, P less than 0.01). The chicken growth rate was therefore used as an accurate predictor of osteomyelitis in individual chickens.