Complement fixation (CF) and hematological studies were performed on 517 cows living in normal conditions in different geographical areas of France. The animals belonged to three different categories: (1) multiple or single case herds, in which lymphosarcomas had been detected in the past five years. (2) Leukemia free herds of high risk regions. (3) Leukemia free and apparently unexposed herds. Positive animals were found with both methods in the first two categories but not in the third. Persistent lymphocytosis and CF antibodies were more frequent in leukemia than in exposed but leukemia free herds. Approximately 2 times more animals were found positive by serologic than by hematologic tests. The mean geometrical titer of CF antibodies was higher in lymphocytotic than in normal animals and highest in lymphosarcomatous cows. Persistent lymphocytosis was first detected in 3-years-old animals whereas 22% of younger cows were positive in the CF test.