Data on cystine leukocyte content are analyzed in cystinotic patients receiving cysteamine as a depleting agent of cellular cystine. During 2 years, 63 measurements of cystine leucocyte content were performed in 15 cystinotic patients, aged 20 months to 22 years, taking daily 40 to 65 mg/kg of cysteamine. An original method was used to measure leukocyte cystine: a binding protein assay with a specific cystine-binding protein from E. coli. Results were taken individually because of various clinical situations in patients. In 8 patients taking cysteamine regularly, 6 hours after a dose, cystine leukocyte content was between 1 and 2 nmol 1/2 cystine/mg protein, about 10 times less than basal values without treatment and 5 to 10 times more than control subjects. In less compliant patients, cystine leukocyte content was close to basal values without treatment (3 to 25 nmol/mg). Some variability was observed between individuals receiving cysteamine: pharmacokinetic parameters may need further investigation. This sensitive and specific assay helped in controlling compliance and adjusting dosage regimen in each patient. The aim was to maintain a minimum effective dosage in order to avoid toxic side effects of cysteamine.