We present two cases of hypersensitivity to cefazolin (one anaphylaxis). The skin prick tests with cefazolin at therapeutic concentration were positive. For the in vitro study we used conjugates of cefazolin with human serum albumin, obtaining a positive result in the histamine release test in both patients. In one case, we demonstrated anticefazolin-specific IgE antibodies by RAST. The controls did not react to any of these tests. Finally, we carried out skin tests and challenge tests with other betalactam antibiotics, including benzylpenicillin, amoxicillin, aztreonam, cefuroxime and cefotaxime, showing in both patients good tolerance to all these drugs.