We have described a patient with Klebsiella pneumoniae meningitis who was treated with cefotaxime and chloramphenicol concomitantly, and whose slow initial resolution and subsequent relapse plus in vitro evidence of antagonism of cefotaxime appear to indicate that chloramphenicol interfered with the activity of the cephalosporin. Thus, concomitant use of chloramphenicol should probably be avoided or used advisedly in adults with gram-negative bacillary meningitis susceptible to a third generation cephalosporin.