Comparative studies aiming at the detection of certain tubular protein elements by means of Ouchterlony's immunodiffusion, in parallel with lysozyme and guanase assays, were carried out in the unconcentrated urines of 746 subjects, of whom 655 apparently healthy inhabitants (mostly children) from a region with endemic nephropathy (EN) and from Bucharest, as well as 91 adults with EN or various other diseases with renal involvement. The presence of light chains, of lysozymuria exceeding 2 microgram/ml, of beta2 microglobulin and of guanase in the urines of children and adults from the endemic area was significantly more frequent than in the control groups. These immunochemical changes are hence considered as valuable criteria for the detection of EN prior to the uremic stage. They should be looked for, first and foremost, in the young relatives of patients with this disease. In the stage of nitrogen retention the diagnostic value of these tests is reduced, since the same changes can also be found in the urines of patients suffering from other diseases with renal involvement, which show nitrogen retention as well.