The complement-fixation in drops, using a triple antigen, was employed for screening out sera from Chagas' disease, syphilis or brucellosis. A total of 112, 365 sera were tested, from which 18,279 reacted with the triple antigen (composed of cardiolipin, suspension of brucellas and T. cruzi antigen). Alt the reactive sera were retested with each one of the antigens, in order to identify the infection. Chagas' disease was found in the proportion of 77% to 88% in this group, syphilis from 8% to 23% and 1% to 3% reacted with brucella antigen. The use of a triple antigen in the complement-fixation test in drops has been very suitable for screening out reacting sera for Chagas' disease, syphilis and brucellosis, not only for the selection of blood donors, but also for the routine of serology in public health laboratories and hospitais, when large number of sera are to be tested.