Due to the recent increasing mortality rates in 1- to 4-month-old Thoroughbred foals with Rhodococcus equi infection in Turkey, serum samples from 454 foals between 30 and 60 days on studfarms of the Marmara Region, where the Thoroughbred foal population is concentrated, were investigated by ELISA for early diagnosis of the infection. The Tween 20-extracted antigen of the R. equi ATCC 6939 strain was used in the ELISA. Two hundred and nine (46%) of the 454 foals were found to be ELISA-positive. Optical density (OD) values of 0.3 (positive limit) or higher were detected in 43% of foals aged 30-40 days, 40% of those aged 4150 days and 78% of those aged 51-60 days. Of the 209 ELISA-positive foals, 126 (60%) showed a variety of clinical signs. All ELISA-positive foals were treated with a combination of erythromycin and rifampin. Of the 126 ELISA-positive foals with clinical signs, 122 (97%) recovered, but 4 of the foals died during the treatment period. The OD values of the 4 dead foals were over 0.3. Necropsy confirmed that they had R. equi infection. Three of the 4 had lung abscesses and the remaining had abdominal abscesses. In addition, 2 foals died on 2 different studfarms unrelated with this ELISA surveillance study, and they had lung abscesses. Virulent R. equi was isolated from lesions of the 6 dead foals. All the isolates had 15- to 17-kDa antigens and 85 kb type-I virulence plasmids. These results indicated that virulent R. equi, which contained 85 kb type-I plasmid, is widespread in Thoroughbred foals in the Marmara Region and ELISA is useful for the early diagnosis of R. equi infection in foals.