There is increasing concern over the potential inhalation hazard to health care workers from blood-borne pathogens. Previous studies have demonstrated that inhalable, blood-associated aerosols are produced during orthopedic surgery. However, the identification and quantitative estimation of blood-associated aerosols have been based upon simple ＇＇dipstick＇＇ analysis of the collected samples. In order to confirm the presence of these blood-associated aerosols and to estimate the amount produced, the red blood cells of five dogs were radiolabeled with Cr-51 before aerosol samples were taken during total hip replacement procedures. Various aerosol sampling devices including a Marple personal cascade impactor, two Lovelace multi-jet impactors, and air filters were used. The Marple personal cascade impactor was worn by the surgeon. One Lovelave multi-jet impactor was sampled near the surgical site, while the other Lovelace impactor and the filter samples were taken through a probe located near the surgical site. The samples were subjected to gravimetric analyses, and blood contents were assessed by Chemstrip 9 analysis and by radioactivity counting. Results confirmed that blood-associated aerosols were produced during orthopedic surgery. The time-averaged mass concentration near the surgical site, as measured by the personal impactor, was 0.37 mg m(-3);of that amount, 6.5 mu g m(-3) (1.8 % of the total mass concentration) was attributed to red blood cells (RBCs). The estimated number of RBCs or hemoglobin that might be inhaled by a surgeon without any respiratory protection during the course of an orthopedic surgery was about 2.9 x 10(5) RBCs or 8.7 mu g of hemoglobin. About 60% of the RBCs were associated with particles larger than 10 mu m in aerodynamic diameter, and about 8% of the RBCs were associated with particles less that 0.5 mu m. The number ratio between the RBCs and lymphocytes for humans is about 2200:1; thus, the estimated number of lymphocytes that might be inhaled by a surgeon without any respiratory protection during the course of an orthopedic surgery would be less than 135. To assess the significance of our finding on the potential risk to health care workers will require further studies of the relationship between pathogens and particle sizes and the viablity of pathogens associated with these blood-associated aerosols.