Plasma concentrations of the acute phase proteins (APP), C-reactive protein (CRP) and haptoglobin (Hp), increased markedly following experimental infection of dogs with Trypanosoma brucei. The highest concentrations of CRP were observed immediately after peaks of parasitaemia. Treatment with curative doses of the trypanocidal drug suramin caused a rapid decrease in CRP. Relapse infections after subcurative treatment were followed by a reappearance of high plasma CRP concentrations. Haptoglobin remained elevated during the course of the disease. Curative treatment with suramin caused a gradual but slow decrease in Hp while subcurative treatment caused no significant changes. Thus, the estimation of CRP was useful in determining the presence of active infection and the success of chemotherapy. High Hp levels in severely anaemic dogs indicated that intravascular haemolysis does not contribute significantly to the anaemia associated with T. brucei infections in dogs. These conclusions need confirmation from a larger experiment.