This report focuses on angiotensin II AT₁ receptor autoantibodies (anti-AT₁-AABs) in preeclamptic women. An enzyme-linked immunosorbent assay was described. Biotinylated peptide was incubated with anti-AT₁-AABs. Streptavidin-coated magnetic particles bind the protein–autoantibody complex. Detection of anti-AT₁-AABs was performed using anti-human IgG3 peroxidase-coupled antibody. The color reaction of tetramethylbenzidine solution was stopped by adding 0.5 M H₂SO₄. Optical density was measured at 450 nm (620 nm reference filter). Seventy-nine percent of anti-AT₁-AAB–positive patients (measured by bioassay) showed an increase in optical density (>145%). The same biotinylated peptide was successfully used for purification of 6/6 anti-AT₁-AABs. Chronotropic effects of purified antibodies were registered on primary cultured neonatal rat cardiomyocytes with the computer imaging system IMAGOQUANT. Western blot of coimmunoprecipitation of angiotensin II AT₁ receptor shows one band (molecular weight >40.0 kDa) in potassium thiocyanate eluate.