Rat thyroid FRTL-5 cells were employed to study the synthesis and degradation of a functional integral lysosomal membrane protein, the lysosomal cystine transporter. This carrier exhibited countertransport and closely resembled the cystine transport system of human leucocytes, fibroblasts, and lymphoblasts shown to be defective in the lysosomal storage disease, nephropathic cystinosis. Using cycloheximide to prevent new protein synthesis, the half-life of the FRTL-5 cell lysosomal cystine carrier was determined to approximate 21 h. Carrier function was not influenced by the N-glycosylation inhibitor tunicamycin, nor by the oligosaccharide processing inhibitors castanospermine and deoxymannojirimycin. The data suggest that the lysosomal cystine carrier is a protein without strict functional requirements for N-linked oligosaccharides, and that rat FRTL-5 cells can be employed in future investigations into the structure and function of other integral lysosomal membrane proteins as well.