With 6.5 million obese recorded in France in 2009, obesity is a major public health: it is a chronic disease associated with many respiratory, cardiac, metabolic, musculoskeletal complications. The risk of mortality and morbidity are directly proportional to the importance of overweight and medical treatment alone is only moderately effective weight loss. Bariatric surgery is the only treatment with proven efficacy in patients with a body mass index above 40 kg/m2. As with any surgery, there is a risk of surgical site infection (SSI). The SSI are the third most common nosocomial infection after urinary tract infections and airway. The fight against the ISO requires the administration of antibiotic prophylaxis acting on the main bacteria found in bariatric surgery. There are several studies in the literature interested in the dose of cefazolin in bariatric surgery. However, no published pharmacokinetic studies defines an optimal dose to obtain tissue and plasmatic concentrations of cefazolin higher than minimum inhibitory concentrations of the main germs encountered in these surgery. Only empirical recommendations are published, including the SFAR and the National Institute of Health in 2010. This study aims to determine whether the dose of 4 grams of cefazolin can achieve these goals of concentration and estimate an injection time of preoperative ideal for an adequate tissue concentration at the time of the incision.