Anti-gamma-globulin activity, free DNA, and DNA binding were studied in 19 cryoglobulinemias: 8 patients with Waldenström's disease (WD), 9 patients with essential cryoglobulinemia (EC), and 2 with chronic aggressive hepatitis (CAH). Antihuman gamma-globulin activity was detected in all sera and dissolved cryoprecipitates but two from EC and one from the CAH group. By diphenylamine assay we found DNA in two sera from WD and in one serum and cryoprecipitate from EC. An antibody to denaturated DNA was shown only in sera from the two patients with CAH and from one patient with EC. Nonspecific binding was more frequent.