The temperature records of 30 patients were examined for a febrile response after the injection of 111ln-DTPA for cisternography. All lots of the material were tested and found to be negative by the U.S. Pharmacopeia rabbit test for pyrogens. The material also was tested for reactivity with the amebocyte lysate of Limulus. Six of the seven patients who received material strongly reactive with the Limulus lysate had temperature increases greater than 1°F within eight hours of the injection. No febrile response was observed in the remaining patients, who received material weakly reactive or nonreactive with the Limulus lysate. In two of the seven patients who received the strongly reactive material, aseptic meningitis also developed. This observation supports the hypothesis that bacterial endotoxin contamination of radiopharmaceuticals used in cisternography may cause aseptic meningitis.