In African trypanosomiasis the occurrence of antibodies to interferon‐γ (IFN‐γ) was studied in both humans and experimental rats. Sera from patients infected withTrypanosoma brucei gambienseshowed increased levels of antibodies to IFN‐γ as compared with controls from the same regions in Africa. In Sprague‐Dawley rats infected withTrypanosoma brucei bruceian early appearance of IFN‐γ‐producing spleen cells was observed, followed by an increase in levels of antibodies against IFN‐γ in the sera. Previously, IFN‐γ has been found to play a crucial role in trypanosome infections in rats by promoting proliferation ofTrypanosoma brucei brucei. The appearance of antibodies to IFN‐γ in humans, as in rats, indicates that this cytokine is produced also in the human infection. Its parasitic growthstimulating and pathophysiological effects on the organism may be r