A total of 2530 sera were obtained from 811 beef herds undergoing VIA testing during 1992, belonging to Buenos Aires province (the main cattle breeding province with 20 million heads). The districts sampled were: Ayacucho (n=223); Balcarce (n=96); Alvear (n=247); Bueyrredo'n (n=250); Las Flores (n=235); Maipu (n=258); Magdalena (n=388); Mar Chiquita (n=230); Olavarria (n=243); Rauch (n=138) and Tapalque (n=232). The original design was a district stratified random sample (expected prevalence 50%, error 5%) but the final number of samples was less since only cattle over 24 months of age were studied. The sera were tested for antibodies to Mycobacterium paratuberculosis using an indirect ELISA test performed as described by the supplier (Allied Monitor, Lafayette, MO, USA). The adjusted prevalences (using a 58.8% sensitivity and 95% specificity) and their standard errors were as follows: Ayacucho: 17.5 (±2.5); Balcarce: 51.5 (±1.6); Alvear: 7.9 (±1.7); Pueyrredon: 2.5 (±3.1); Las Flores: 23.6 (±2.8); Maipu: 13.6 (±2.1); Magdalena: 23.8 (±2.2); Mar Chiquita: 15.5 (±2.4); Olavarria: 12.9 (±2.1); Rauch: 28.9 (±1.2) and Tapalque': 4.2% (±1.3%). The high prevalences in some districts are similar to the figures found in Canada and USA, there are no similar data from Argentina to compare with. Johne's disease is an emerging and apparently widespread disease in our region which requires the implementation of a control program.