The described ELISA reaction is suggested as a method for examination of antitoxoplasmatic IgG antibodies. It is a simple, i.e. three-layer sandwich with the following layers: 1. antigen 2. the examined serum either in serial dilution or single dilution 1:300 and 3. a peroxidase anti-IgG conjugate with a final conclusive step, i.e. a substrate solution with OPD. Sera and conjugate are allowed to bind for 30 mins. at 37 degrees C and the substrate solution with OPD is allowed to act for 15 mins., also at 37 degrees C, and thus the entire reaction lasts about two and half hours. The authors describe alternative procedures with standard serum of a known number of international units (I.U.) or without it, evaluated optically or by a photometer. In the version with standard serum evaluated optically we compare the different localization of equally stained wells in the series of serially diluted (4n) examined and standard serum and thus we assess, based on the different dilutions the number of I.U. in the examined serum. If we compare the sera with the serum standard by means of a photometer, we examine them in a single dilution of 1:300 and compare the resulting optical density (OD) with the calibration curve made with four dilutions of the standard. The calibration curve is plotted on a scale which straightens it approximately at an angle close to the optimal 45 degrees and which makes it possible to take readings of interpolated international units.(ABSTRACT TRUNCATED AT 250 WORDS)