ABSTRACT. Silva F.J.M., Silva J.B., Guedes Júnior D.S., Rangel C.P., Teixeira R.C., Fonseca A.H. & Faccini J.L.H. [Detection of IgG antibodies to Anaplasma marginale in newborn calves in Baixada Fluminense region, RJ.] Detec- ção de anticorpos IgG anti-Anaplasma marginale em bezerros recém-nascidos na Baixada Fluminense, RJ. Revista Brasileira de Medicina Veterinária, 37(2):133-137, 2015. Curso de Pós-Graduação em Ciências Veterinárias, Instituto de Veteriná- ria, Universidade Federal Rural do Rio de Janeiro, BR 465 Km 7, Seropédica, RJ 23890-000, Brasil. E-mail: firstname.lastname@example.org The objectives were to evaluate the prevalence and the seroepidemiological evolution to Anaplasma marginale in 41 calves since birth to first 180 days of life, in a enzootic region in the state of Rio de Janeiro. The analyzed animals belonged to Centro Estadual de Pesquisa em Agricultura Orgânica - Pesagro-Rio, Seropédica-RJ. The study was conducted during the rainy and dry seasons, between 2009 and 2010. A total of 1607 serum samples was collected initially every three days and processed using indirect ELISA test to detect IgG anti-A. marginale antibodies. Percent values for A. marginale seroprevalence as function of age were tested using the χ2 test at 5% significance level. The prevalence of IgG anti-A. marginale antibodies were 39.8% in calves younger than 30 days, 23.3% between 30 and 60 days, 27.3% for 60 and 120 days and 38.2% to 120 and 180 days. Calves with age between 30 and 60, 60 and 120 and 120 and 180 days were respectively 1.90, 1.75 and 1.55 more risk to be seronegative for A. marginale than newborn ones. The results show that calves had low levels of IgG antibodies anti-A marginale in all study, this condition induces them to development of clinical anaplasmosis. Moreover, the property was considered unstable epidemiologically to A. marginale, although located in a endemic stability area.