A new animal model of invasive aspergillosis is described in which female New Zealand White rabbits were immunosuppressed with corticosteroids and cyclophosphamide and were given an intratracheal inoculation of 4 × 104 conidia of Aspergillus fumigatus. Thirteen of 15 animals survived during a 10-day-period of observation. Most had clinical signs of a respiratory infection (dyspnoea) and at autopsy there was macroscopic and microscopic evidence of invasive pulmonary aspergillosis. Six control animals (infected but not immunosuppressed) showed no such signs. The extent of hyphal invasion was assessed histologically and quantified by calculating the number of colony forming units (c.f.u.) g-1 of tissue: in the experimental group the mean c.f.u. value for the lungs was 1·25 × 103 compared to 73·3 c.f.u. g-1 of lung for the control group (P=0·003). The infection was also quantified by a whole lung chitin assay: in the experimental group the mean chitin content (expressed as a glucosamine equivalent) was 3·05 μg g-1 lung tissue compared to a 0·53 μg g-1 lung tissue for the control group (P=0·01). We conclude that this model of invasive aspergillosis in rabbits reflects closely the pathological features of the disease in man and that it may prove useful for studies of the pathogenesis and the treatment of invasive aspergillosis.