Examples of already-characterized parasite T-cell epitopes (1 –5 ; see also refs. in 6 ) fit with current models of antigen recognition, in which T-cells recognize foreign proteins only in the form of peptides associated with major histocompatibility complex (MHC) antigens. Peptides bound to Class I or Class II MHC proteins are recognized, at the surface of an antigen-presenting cell (APC), by T-cells carrying the CD8 or CD4 molecules, respectively. Recognition through the T-cell receptor (TCR) leads to activation of the T-cell. Activated CD8++ T-lymphocytes are cytotoxic, and lyse target cells presenting the recognized peptide and MHC Class I antigen. Cloned murine, and more recently human, CD4++ lymphocytes have been classified as TH1 cells, which secrete γ-interferon, resulting in macrophage activation, or TH2 cells, which secrete interleukin-4 (IL4), resulting in B-cell differentiation and antibody secretion (7 , 8 ). Experiments on T-cells recognizing Leishmania antigens were important in establishing properties of these cell types (9 , 10 ).