Aflatoxin M-1 (AFM(1)) levels were determined in 115 raw cow milk samples collected from plain and mountain villages during March-April 2003 in Bursa province, Turkey. The samples were analysed by competitive enzyme-linked immunosorbent assay (ELISA) technique. The mean concentration of AFM(1) was determined as 78.06 +/- 6.34 ng/kg (0.00-212.40 ng/kg) in milk samples of plain villages and 71.72 +/- 5.52 ng/kg (12.30-164.10 ng/kg) in those of mountain villages. Although the mean concentration of AFM(1) in the plain villages' milk samples was higher than in those from the mountain villages, this difference was not found statistically significant. Overall, the AFM(1) incidence in all collected milk samples, regardless of their sources was found noticeably high (99.13%) with approximately 60% of the samples exceeding the European Union (EU) and Turkish tolerance limit of 50 ng/kg. The AFM(1) concentrations in milk samples exceeding the indicated limits with respect to their source of collection were determined as 61.82% in plain and 56.67% in mountain villages.