Sheep red cells, sensitized with a genus-specific leptospiral substance extracted from a water leptospira, were preserved by freeze drying. The serological reactivity of the preparation was tested against a total of 41 sera from proven cases of human leptospirosis and 27 from proven cases of animal leptospirosis. The sensitized, freeze-dried red cells appeared to be a sensitive reagent for the detection of leptospiral antibodies by haemagglutination. The use of this reagent as a screening test for leptospiral diagnosis is discussed. In the reported haemagglutination test on human and rabbit antisera the findings suggested that only IgM (19S) antibodies took part in the reaction.