We suggest that countercurrent isotachophoresis performed on cellulose acetate membranes (ITP-CAM) should be used for detecting trace amounts of Bence-Jones protein (BJP) in urine of patients with chronic lymphocytic leukemia (CLL), non-Hodgkin's lymphoma (NHL) and related diseases. ITP-CAM allows simultaneous concentration and electrophoretic separation of proteins present in highly diluted solutions, as well as easy immunological detection of separated substances. BJP was found in 24 out of 42 patients with CLL, 33 of 56 with NHL and 3 of 3 with Waldenström macroglobulinemia. Twenty-three patients were followed during the course of chemo- or radiotherapy. In 19 cases the BJP findings correlated well with clinical status. In no case of partial or complete clinical response did BJP completely disappear from the urine.