A way to study the role of intracellular trafficking of an antigen in its presentation to T cells is to target the antigen to various cell compartments of the antigen‐presenting cells (APC) and compare the nature of the complexes associating major histocompatibility complex (MHC) molecules and antigenic peptides, expressed on the cell surface. MHC class I+ and MHC class II+ mouse L fibroblasts secreting hen egg lysozyme (HELs cells) or expressing HEL in their cytosol (HELc cells) were obtained after transfection with HEL cDNA and signal sequence‐deleted HEL cDNA, respectively. HEL was evidenced in both HELs‐and HELc‐transfected cells and the former type of transfectant secreted a large amount of HEL. However, HEL produced in the cytosol exhibited a short half‐life of less than 5 min. HEL‐derived peptides could not be shown biochemically either in HELc‐ nor in HELs‐transfected cells. We then studied the capacity of these cells to present HEL to HEL‐specific class I‐ and class II‐restricted T cells. Both cell types could be recognized by the HEL‐specific MHC class I‐restricted CTL clones. In contrast, MHC class II‐HEL peptide complexes, recognized by HEL‐specific helper T cell hybridomas, could be detected on MHC class II+ HELs‐ but not HELc‐transfected cells. In vivo experiments showed, however, that HELc‐transfected cells could provide host APC with HELc‐derived peptides able to associate with MHC class II molecules. This was inferred from the capacity of MHC class II− HELc‐transfected cells, unable by themselves to elicit any anti‐HEL antibody response, to prime syngeneic and allogeneic mice against HEL. The priming was revealed by the induction of an antibody response after a boost with an amount of HEL unable itself to elicit an antibody response.