Trypanosoma brucei brucei anTAT 1.1E parasites purified from the blood of infected mice, fixed with paraformaldehyde, incubated first with a murine monoclonal antibody elicited against recombinant trypanosome-suppressive immunomodulating factor and then with fluorescein isothiocyanate–labeled antimouse IgG, and visualized by means of a Zeiss microscope. The nucleus and kinetoplast of the parasite were stained in blue with 4
,6-diamidino-2- phenylindole. Trypanosome-suppressive immunomodulating factor, a newly identified parasite molecule inducing classically activated macrophages secreting nitric oxide and tumor necrosis factor, has the potential to contribute to the inflammatory response required to control parasite growth and to the pathogenicity of African trypanosomiasis. (See Go´mez-Rodri´guez et al, on pp 1849–60.)