Levels of C-reactive protein and lactate were determined on 562 consecutive cerebrospinal fluid (CSF) samples from adult patients with a wide variety of central nervous system diseases to compare the sensitivity and specificity of CSF lactate and C-reactive protein for the rapid diagnosis of bacterial meningitis. Neither test alone, together, or in combination with elevated CSF leukocyte count and protein had a predictive value over 60% for a positive test in this group of patients with diverse central nervous system problems. Neither test is useful as a screening test for bacterial or mycotic meningitis. Also, in patients with partially treated bacterial meningitis, the tests are often negative. CSF lactate may be useful in differentiating aseptic from septic meningitis in selected patients.