The endermic reaction was positive in 10% of 345 patients tested with Bacillus abortus antigens. The positive group exhibited severe local and constitutional reactions when tested with a heat-killed B. abortus antigen. With a "fat-free" antigen the local and general reactions were considerably less. With the former antigen, high anti-abortus agglutinin titers were induced in both positive and negative reactors by a single skin-test dose. With fat-free antigen, agglutinin production was negligible. Fat-free abortus antigen is recommended in cases of suspected undulant fever without agglutinins for B. abortus.