By means of established single-diffusion techniques, immunoglobulin M (IgM) levels in the blood and cerebrospinal fluid of patients with confirmed Trypanosoma rhodesiense infections were studied at the time of parasitological diagnosis, while the patients were being treated with suramin and melarsoprol, and during post-treatment surveillance. During the 3-year study period the number of diagnosed cases in which there was a raised level of IgM in the blood varied considerably, depending on whether diagnosis was made early or late in the course of the infection. In some cases the blood IgM levels returned to normal during preliminary courses of treatment with suramin and in others by the end of a 1-month course of treatment with melarsoprol, while in some other cases they did not return to normal for about 2 years. Similar results were obtained for IgM in cerebrospinal fluid, except that if IgM could be detected at the start of treatment with melarsoprol it was still detectable at the end of treatment. However, cerebrospinal IgM usually disappeared after 2 years. The results indicate that IgM levels are a useful indicator in the diagnosis of the more chronic cases.