Immunogenicity and property of antigens obtained from Trypanosoma gambiense-infected mouse blood (IMP) were examined. A strong vaccine effect against intravenous challenges with 3 x10(3) parasites given on study day 3, 5, or 14 (day 0 = immunization) was observed in mice immunized with a combination of IMP (2 mg protein/mouse) and Freund's complete adjuvant (FCA). But when the challenge was given on day 21 or 30, per cent survival in mice dropped to the 20- and 40-per cent level, respectively. Among fractioned components of IMP, IMP-1, IMP-2, and imp-3, by gel filtration with Sephadex G-200, all of the mice immunized with IMP-1 antigen alone or together with FCA and challenged on day 5 were able to conquer intraperitoneal challenges with 1 x10(2) parasites. Mice immunized with IMP-2 or IMP-3 died within 6 days after challenge. Moreover, protection efficacy shown by IMP-1p (144,000 xg sediment of IMP-1) antigen in mice was similar to that by IMP and IMP-1 antigens. IMP-3 yielded a single precipitin line against mouse anti-IMP serum by Ouchterlony double diffusion method but this response was eliminated when the antiserum was absorbed by IMP-1p. No precipitin line was identified between mouse anti-IMP serum and IMP-1 or IMP-2. From electron microscopic observations, elements of IMP-1 and IMP-1p are possibly corresponded to the fragments of filopodia of the parasites.