A microagglutination test for the detection of Brucella canis antibodies in humans is described. The use of safranin-dyed B. canis organisms as antigen gave easily interpretable settling patterns and reproducible endpoints. The finding of 18 of 1,147 sera with titers greater than or equal to 1:160 resulted in a presumed false-positive rate of 1.6%. Repeat testing of these positive sera in diluent to which 0.6 m NaCl had been added showed a decrease in titer to less than or equal to 1:40 in eight. Conversely, the increased salt concentration permitted detection of blocking antibodies in two sera which were originally thought to be negative. The test provided serologic evidence for B. canis infection in four patients with otherwise undiagnosed febrile illness. The disease is underdiagnosed due to general lack of serologic testing facilities and misconceptions concerning its prevalence.