SummaryThe free (nonprotein) cystine content of human cystinotic fibroblasts was found to vary with the pH of their culture medium. Intracellular cystine content was highest at alkaline medium pH's and lowest at acidic medium pH's. When maintained in medium of pH 6.3, cystinotic fibroblasts lost 50% of their free-cystuie content in 6 hr and over 95% in 30 hr. Cystine was the only amino acid to show significant variation with changes in the pH of the culture medium.