An increased IgM concentration in cerebrospinal fluid (CSF), occurring as a consequence of massive intrathecal IgM synthesis, is a marker of interest for diagnosis of the meningo-encephalitic stage in human African trypanosomiasis. However, in current practice, IgM in CSF is not determined because of the lack of a simple and robust test that is applicable in African rural regions where the disease prevails. We describe the development of a sensitive semiquantitative card agglutination test, LATEX/IgM, for IgM quantification in CSF. The test is simple and fast and the lyophilized reagent remains stable even at 45 degrees C. CSF end-titres obtained with LATEX/IgM parallel the IgM concentrations determined by nephelometry and enzyme-linked immunosorbent assay. Detection of intrathecal IgM synthesis is the most sensitive marker for CNS involvement in sleeping sickness. At a cut-off value of >or= 8, the sensitivity and specificity of LATEX/IgM for intrathecal IgM synthesis are 89.4 and 92.7%. As a consequence, patients with LATEX/IgM end-titres >or= 8 are likely to have intrathecal IgM synthesis, thus central nervous system involvement and therefore should be treated accordingly. Further studies should concentrate on the relationship between the LATEX/IgM end-titres, presence of intrathecal IgM synthesis and occurrence of treatment failures in patients treated with pentamidine.