Cystinotic fibroblasts contain highly elevated amounts of intracellular non-protein cystine reaccumulation by cystine-depleted cystinotic fibroblasts and the steady-state cystine content of nondepleted cystinotic fibroblasts can be modulated by the addition of bovine serum albumin to the culture medium. This effect is not seen in cultures of normal and cystinotic heterozygote fibroblasts. The cystinotic homozygote cells accumulate cystine under there conditions from proteolysis of the albumin. An increased rate of pinocytosis or proteolysis of albumin does not account for the observed cystine accumulation by the cystinotic fibroblasts. Comparison of the amount of cystine accumulated to the amount of albumin degraded shows that less than one percent of the cystine moieties released by proteolysis is retained within these cells.