The complement-fixation test, or, as it is known more popularly, the Wassermann reaction, was at one time considered the only reliable laboratory test for the diagnosis of neuro-syphilis. It is a complicated biological reaction, and requires expert knowledge, theoretical and practical, before any deductions can be made as to whether a patient suffers from neuro-syphilis or not. In the majority of cases the cerebro-spinal fluid has to be sent to a special pathologist attached to some general hospital, and in many cases it means sending the fluid to other towns.