Abstract A method for performing complement fixation tests and antistreptolysin “0” titrations in transparent plastic sheets is described. The sheets are provided with cylindrical flat-bottomed cups. Details of the method are presented. The complement fixation test is carried out in accordance with the Kolmer technique except in the varying of some minor details. A modification of the AS “0” titration described by Kalbak is applied. The use of sheets saves a considerable amount of space and time not associated with a loss of accuracy. The titers of positive sera found with the aid of sheets are as equally high as those obtained with the aid of test tubes.