The serum-protein and -lipoprotein patterns of rats, given a purified diet containing 0%, 0.005% or 0.03% 2-acetamidofluorene, have been determined by disk electrophoresis. 2. An abnormal high-density lipoprotein was detected from the first week of the experiment in the serum of the animals given the carcinogen. The low-density lipoproteins also increased on prolonged feeding with the carcinogen.