When a patient with bacterial meningitis, bacterial endocarditis, or staphylococcal septicemia is suspected or proved to have severe penicillin allergy, treatment with an antimicrobial agent other than penicillin or one of its semisynthetic analogues may be warranted. Chloramphenicol sodium succinate can be substituted for a penicillin in the treatment of pneumococcal, meningococcic, or Hemophilus influenzae meningitis. Vancomycin hydrochloride can be prescribed as an alternate drug for treatment of enterococcal endocarditis. Vancomycin or cephalothin sodium is recommended for bacterial endocarditis caused by penicillin-susceptible streptococci or for staphylococcal septicemia. Substitutions for a penicillin should be made only if life-threatening penicillin hypersensitivity clearly is present, or as initial therapy in an emergency until time allows allergy study in a patient suspected of being penicillin-sensitive. Tube dilution susceptibility studies and in vitro tests indicatng adequate blood or cerebrospinal fluid antimicrobial activity are required for optimal management.