The most frequently used method for the detection of leucocyte antibodies in human sera is now the agglutination reaction described by Moeschlin and Wagner and Dausset and Nenna. Apart from leucoagglutination, the techniques of leucolysis, leuco-opsonization and leucoprecipitation have also been used. None of these tests has found an application comparable to that of leucoagglutination. The aim of the present paper is to describe the use of the complement fixation test for the detection of leucocyte antibodies in human sera. Preliminary studies on the recovery of antigen from normal leucocytes failed to give satisfactory results, since leucocytes could not be obtained in sufficiently dense and erythrocyte-free suspension; leukaemic leucocytes were therefore used.