The Cuenca Lechera Mar y Sierras (CLMS) includes about 300 dairy farms located in the counties of Tandil, Balcarce, Juarez, Ayacucho, General Pueyrredón, Gonzalez Chavez and Necochea, in the province of Buenos Aires. The purpose of this investigation was to determine the prevalence of infection caused by Bovine Leukemia Virus (BLV) in the CLMS. We investigated the presence of anti-BLV antibodies in 4,203 milk samples taken from 73 dairy farms belonging to the CLMS. An indirect ELISA, which is described and evaluated in this paper, was used to test the antibodies in milk. We classified the dairy farms according to their rate of infection. The percentage of dairy farms free of infection resulted in 31.50. On the other hand, 49.40% of the dairy farms showed a figure between 1% and 15% of infected cattle; 17.80% between 16% and 30%, and the remaining 1.30% turned out more than 30% of infected cattle. If compared with data obtained in the 1979-1981 period, which showed that 95.65% of the dairy farms was BLV-free, it is clear that a dramatic progress of the BLV infection has occurred for the last 15 years. Nevertheless, the CLMS is in a privileged position so as to incorporate an inexpensive control plan to eradicate the BLV infection, as almost 1/3 of its dairy farms is still BLV-free and 49.40% still has a low rate of BLV infection. Only about 20% of the dairy farms would require costly strategies of control.