In the frame of an four-year ecopathological study in 47 intensive dairy farms from Britain (France), a milk simple for bacteriological analysis has been done on each cow at the beginning of the lactation. As the whole, 7852 analysis have been realized. The purpose of the present paper is to describe the results of these analysis and to study some factors as space and time. 53 % of the samples contain pathogen germs. Minor pathogens (Staphylococcus coagulase, bacillus, C. bovis) are dominant, but major pathogens (S. aureus, S. uberis, S. dysgalactiae, E. coli, others streptococcus) are present in one third of non-sterile samples. Milk has been samples in a intervalle of 3 days to 7 weeks after calving. This does not get an important bias, because the variation of the rate is not significant. Intramammary infections (IMI) due to major pathogens increase with the parity. The winter time is defavourable, but there is a higher in june for major pathogens. There is a year effect, the rate of IMI having a trend to decrease. Finistère is more affected (36.5 % of IMI with major pathogens) than Morbihan (20.9 %) and Ille-et-Vilaine (13.7 %). If we consider only early mastitis and two first milking controle (Somatic cell count-SCC-registration), we observe a higher incidence of mastitis (20.4 %) and SCC > 4 x 10 6 (43.3 %) with major pathogens than with minor (respectively 10.9 and 17.8 %). The rule of farming pratices and environmental conditions will be taken in account in a second step to determine the risk factors of IMI.