In order to assess the Rhodococcus equi infection in three provinces of Turkey (Bursa, Izmir and Istanbul), 696 sera from healthy foals and adult horses were tested by indirect ELISA using a R. equi reference strain (ATCC 6939) as antigen. 103 sera (14.80%) with titres > 0.646 resulted positive. Seroprevalence was significantly higher (P = 0.0053) in male than in female horses of Istanbul province, although higher antibody titres (mean value) were observed in the female group of Bursa and Izmir provinces with differences estimated between provinces (P = 0.0002). Seroprevalence was correlated with age: foals aged less than 1 year (p < 10-4) and horses from 5 to 10 years old (P = 0.018) resulted more infected in Bursa and Izmir provinces. Our findings indicate that R. equi infection actually occurs in all investigated provinces, suggesting the importance of scrological survey to diagnose the infection and to prevent the zoonotic risk. (c) 2006 Elsevier Ltd. All rights reserved.