A study of blood aerosols produced during hip replacement surgery in dogs was conducted to characterize more accurately blood associated aerosols that might be produced during orthopedic surgical procedures. Five adult Beagle-dogs were injected intravenously with blood containing chromium-51 (Cr51) tagged red blood cells (RBCs). The dogs then underwent a total hip replacement. Aerosols generated by the procedure were sampled by a personal cascade impactor worn by the chief surgeon in his breathing zone and two multijet impactors and three air filters placed near the surgical site. The samples were analyzed for total aerosol mass collected, the proportion of RBCs in the samples (determined from the Cr51 label), and the particle size distribution of the aerosols. The samples were also tested with a Chemstrip-9, a hemoglobin detector. The time averaged total aerosol mass concentration collected in the surgeons breathing zone was 0.37mg/m3. Of this, 6.5 micrograms per cubic meter (microg/m3), representing about 1.8% of the total aerosol mass concentration, was RBCs. This was equivalent to about 2.9x10(4) RBCs. The Chemstrip-9 results correlated fairly well with the Cr51 activity in the aerosols. The multijet impactor data indicated that about 60% of the RBCs was associated with aerosol particles having aerodynamic diameters larger than 10 micrometers (microm) and 8% with particles having diameters smaller than 0.5microm. It was estimated that a surgeon performing an orthopedic procedure without wearing any respiratory protection would inhale about 2.9x10(5) RBCs, equivalent to 8.7 micrograms of hemoglobin. The number of lymphocytes that would be inhaled was estimated to be 135, based on an RBC/lymphocyte number ratio of 2,200 to 1. The authors conclude that the potential risk of disease transmission from inhaling aerosols produced during orthopedic surgery appears to be very low.