Parturient paresis affects about 10% of all cows. The most common treatment is injection of various Ca-salts and MgCl2 and about 80% recover after repeated injections. The effect of IgG on the Arrhenius plot of bovine erythrocyte AChE was studied. The plot was normal, i.e. biphasic (a broken line) in the presence of IgG from healthy cows. The highest concentration tested was 10 mg/ml. Before one injection of the Ca-Mg solution 5 mg/ml IgG from sera of paretic cows (n = 10) changed the Arrhenius plots to monophasic (linear). Thus, paretic cows have antibodies against AChE. After injection the change to linear plots was observed already with 1 mg/ml IgG. Apparently, the level of antibodies in serum affecting AChE increased about 5-fold. This is considered to be due to dissociation of bound antibodies from various AChEs. All cows recovered after one injection and release of AChE antibodies might be important for the recovery. Incubation of blood from paretic cows (n = 3) with Ca-salts and MgCl2 increased the amount of free AChE-antibodies about twofold; the Arrhenius plots became linear at 2.5 mg/ml IgG compared with 5 mg/ml before the incubation. This should be due to release from AChEs on blood corpuscles as erythrocytes and lymphocytes. But the increase of antibodies after incubation of blood does not account for the whole increase following the treatment of paretic cows. AChE antibodies are probably also dissociated from other sites, such as neuromuscular junctions.