Uponin vitrostimulation with 10 μM ionophore A 23187 for 5 min at 37°C, the generation of leukotriene (LT) C4in polymorphonuclear leukocytes (PMNL) from nine untreated cystinotic children was significantly increased compared with that in eight control children (p< 0.01) and 25 normal adults (p< 0.001) (417.4 ± 70.0versus177.0 ± 30.9 and 164.9 ± 19.5 pmol/1 x 107cells, respectively). Concomitantly with the increased generation of LTC4, LTB4production in PMNL from untreated cystinotic children was decreased compared with controls, whereas the total amount of LTA4derivatives was similar in the three groups. The increase in LTC4production was not related to the number of eosinophils present in the PMNL preparations from cystinotic children, which was similar to that of control subjects. PMNL from cystinotic children treated with cysteamine, an aminothiol compound that decreases the intracellular cystine content, generated smaller amounts of LTC4upon ionophore A 23187 stimulation than PMNL from untreated cystinotic children. In addition, abrogation of the cysteamine treatment for 3 or 4 d led to an increase in LTC4production. These findings suggest that the metabolic abnormalities taking place in infantile cystinosis may favor the biosynthesis of LTC4from PMNL.