Within recent years it has become a recognised practice to perform lumbar puncture on cases suffering from cerebro-spinal disease; the practical value of the procedure is generally admitted as an aid to diagnosis; and in some diseases, such as tetanus and cerebro-spinal meningitis, the operation is resorted to for therapeutic purposes. It must, however, be admitted that it is only in such diseases of the cerebro-spinal system as are due to the presence of well-recognised organisms that the diagnostic value of lumbar puncture approaches scientific accuracy, in such diseases, for example, as tubercular meningitis, cerebro-spinal fever, pneumococcal meningitis, and sleeping-sickness. The value to be attached to an examination of the fluid for the syphilitic reaction is minimised when one considers that for this test the blood-serum is preferable.