Cell-mediated immunity was studied in 25 children with minimal-change nephrotic syndrome in different stages of their disease. This was assessed by delayed cutaneous hypersensitivity reactions, number of T lymphocytes, and blastogenic response to phytohemagglutinin and PPD. Patients with active nephrosis could not become sensitized to dinitrochlorobenzene and had decreased response to common recall antigens. T-cell number ranged within normal values in all nephrotic patients. During active disease, lymphocyte transformation was markedly reduced, but improved considerably when cells were cultured in normal plasma. During remission, patients showed normal proliferative response. Reactivity of normal lymphocytes to both stimulants was inhibited by nephrotic plasma. Our results suggest the existence of a serum factor that affects T-cell function.