['Preeclampsia is a serious disorder of pregnancy characterized by hypertension, proteinuria, edema, and coagulation and vascular abnormalities. At the cellular level, abnormalities include increased calcium concentration in platelets, lymphocytes, and erythrocytes. Recent studies have shown that antibodies directed against angiotensin II type I (AT1) receptors are also highly associated with preeclampsia.', 'We tested the hypothesis that AT1 receptor-agonistic antibodies (AT1-AAs) could activate AT1 receptors, leading to an increased intracellular concentration of free calcium and to downstream activation of Ca2+ signaling pathways. Sera of 30 pregnant patients, 16 diagnosed with severe preeclampsia and 14 normotensive, were examined for the presence of IgG capable of stimulating intracellular Ca2+ mobilization. IgG from all preeclamptic patients activated AT1 receptors and increased intracellular free calcium. In contrast, none of the normotensive individuals had IgG capable of activating AT1 receptors. The specific mobilization of intracellular Ca2+ by AT1-AAs was blocked by losartan, an AT1 receptor antagonist, and by a 7-amino-acid peptide that corresponds to a portion of the second extracellular loop of the AT1 receptor. In addition, we have shown that AT1-AA-stimulated mobilization of intracellular Ca2+ results in the activation of the transcription factor, nuclear factor of activated T cells.'] Do antibodies from preeclamptic patients stimulate increased intracellular Ca2+ mobilization through angiotensin receptor activation?