Different compositions of artificial cerebrospinal fluid (CSF) and the CSF from sleeping sickness patients both with and without late-stage [i.e. central nervous system (CNS)] involvement were evaluated for their abilities to support survival of Trypanosoma brucei brucei. Artificial CSF, containing the major electrolytes, amino acids and carbohydrate components of healthy fluid, was equivalent to the CSF from patients without CNS involvement (survival time 20 hours). Normal CSF does not therefore contain substances which deter the parasite. The CSF from the late-stage patients did not support survival for longer than 10 hours, probably because it contained increased numbers of lymphocytes and antibodies.