Activation of CD4+ T cells requires processing of exogenous protein antigens by antigen-presenting cells (APC). A macrophage hybridoma and B cell lymphoma were comparable in their ability to process hen egg lysozyme (HEL), which involves reduction of its disulfide bonds. The intracellular levels of cysteine and glutathione, major physiological thiols, based on protein content were similar within these cell lines. In addition, the cysteine transport pathway in viable cells was assessed by 35S-cystine uptake. For macrophages, the majority of the radioactivity resided in high density subcellular fractions of Percoll gradients that comigrated with lysosomal beta-galactosidase (beta-gal). Besides the lysosomes, low density fractions cosedimenting with endosomes incorporated the radiolabel in the B cells. Both peaks of radioactivity disappeared when the B cells were incubated with unlabeled carboxymethyl-cysteine (CM-cysteine), a specific competitor of the plasma membrane CG transport system. The distinct gradient profiles of radiolabel uptake in the cells correlated with a difference in their capacity to process the transferrin-lysozyme conjugate (TF-HEL). TF-HEL was significantly more stimulatory than HEL in inducing a HEL-specific T cell response with the B cells as the APC. However, the potencies of TF-HEL and HEL were similar when the macrophages were the APC. Thus, the intracellular location of cysteine transport activity may be cell lineage-dependent, and its presence may, in part, determine whether an organelle is a productive site of processing antigens with disulfide bonds that is necessary for CD4+ cell activation.