A panel of human T cell clones specific for measles virus was characterized and among them fusion protein-specific, DR1-and DP-restricted T cell clones were selected to study the processing and presentation of determinants borne by a viral membrane protein. Using two independent methods to assess the activation of T cells when they encounter antigen-presenting cells, proliferation assay and Ca2+ flux measure by flow cytometry, we show that determinants from the fusion protein of measles virus presented to two DR1-restricted T cell clones have strikingly different processing requirements. While treatment with chloroquine, leupeptin and brefeldin A of antigen-presenting cells infected with the measles virus inhibits presentation of the first determinant, presentation of the second is prevented only by leupeptin but not by chloroquine and brefeldin A. The major histocompatibility complex deletion mutant cell line T2 was transfected with DR alpha and DR1 beta genes to be tested as antigen-presenting cells with the measles virus-specific T cell clones. DR1-transfected T2 cells infected with the measles virus presented the fusion protein determinant whose processing was sensitive to chloroquine and brefeldin A but failed to display insensitivity to these two drugs, further indicating that the two determinants are generated following two distinct pathways. The first is likely to be independent of the expression of the class II major histocompatibility complex-like molecule DM, whereas the other requires it. In conclusion, determinants on the same polypeptide can have profoundly dissimilar processing requirements. Due to transport to successive compartments with different processing capabilities, more determinants are successfully released from antigens and/or captured by class II major histocompatibility complex molecules, thereby increasing the repertoire of determinants displayed by class II major histocompatibility complex molecules.