BACKGROUND Antimicrobial prophylaxis is considered beneficial for preventing surgical-site infections in clean orthopaedic surgery. However, whether tissue concentrations of cefazolin achieve the minimum inhibitory concentration for the targeted contaminants have yet to be clarified. QUESTIONS/PURPOSES We asked whether 2 g of cefazolin would enable effective serum and bone concentrations relative to the current minimum inhibitory concentration for cefazolin-resistant coagulase-negative Staphylococci and methicillin-sensitive Staphylococcus aureus. PATIENTS AND METHODS We enrolled 43 patients (THA, n = 16; TKA, n = 27) scheduled for primary THAs and primary TKAs. Subjects were given 2 g of cefazolin intravenously before incision. One blood sample and two bone samples were collected from each subject before tourniquet deflation before any additional dose. All samples were assayed at the same laboratory. Minimum inhibitory concentration values were defined based on nationwide surveys. RESULTS Mean (± standard deviation) serum concentration was 170.3 ± 51.3 μg/mL (range, 99.3-370.3 μg/mL). Mean bone concentration was 32.3 ± 15.2 μg/g (range, 11.4-70.0 μg/g) in THA, and 16.0 ± 10.4 μg/g (range, 6.3-46.3 μg/g) in TKA. All serum and bone concentrations exceeded the minimum inhibitory concentration for methicillin-sensitive S. aureus, but some serum levels were marginal and no bone levels exceeded the minimum inhibitory concentration for cefazolin-resistant coagulase-negative Staphylococcus. CONCLUSIONS Our data suggest intravenous administration of 2 g of cefazolin achieves the minimum inhibitory concentration for methicillin-sensitive S. aureus in serum and bone, but not the minimum inhibitory concentration for cefazolin-resistant coagulase-negative Staphylococcus in bone, resulting in a potential risk of deep surgical site infections in THAs and TKAs.