In 36 children with nephrotic syndrome (NS) a significant decrease of their serum levels of the thymic-hormone--thymulin--was found, in parallel with a diminution of their mean blood zinc levels as compared to 33 control children. The serum level of thymulin normalized after the in vitro addition of ZnCl2. Similar results have previously been reported in mice subjected to a long term zinc deficient diet. It can therefore be assumed that the decreased biological activity of the thymic hormone noted in NS is secondary to a low serum zinc level. These data could explain certain anomalies of T lymphocytes described in childhood NS.