The work is focused on find out the level of biosecurity in cattle breeding depending on herd size, and on determining basic preventive measures against introducing virus of IBR or BVD into the herd. Forty five dairy farms from all Czech Republic were grouped by the size of the herd into three groups (under 100, 101-500 and above 500 cows). There was collected and analysed a complex of preventive measures designed to prevent the penetration of infectious agents into farm by person, animals, health, transport, technological systems and sanitation. Individual characteristics and overall assessment of biosecurity were evaluated by three-point scale (low, moderate and high level). On the basis of monitoring in dairy cattle farms, it was proved that small herds differed statistically significantly in level biosecurity from farm with medium and large herds. Farms with small herds had a significantly lower level of herd protection than other persons. The possibility of spread of infection into the farm by purchasing animals, including ensuring quarantine, is a problem mainly on the farms with low animal concentration. As well as the question of maintaining the well-health herd through the introduction of preventive and curative measures and their strict adherence, as well as sanitation on the farm.