The effect of the type of stall, number of cows per milker, automatic cup removal, height of milk pipeline, milking time, udder washing practice, teat disinfection after milking, milking speed, drying practices, washing practices of milking parlor and milking equipment and bedding practices on somatic cell count (SCC) were analyzed. Data consisted of 526 lactation records taken in 9 Holstein herds located in the North of Navarra (Spain). Data were transformed to the log scale before parametric analysis. Average SCC was 380.000 cells/ml, with an important variation between herds (185,000‑618,000). Average milk production was 7,845 kg with 3.72 and 3.16 fat and protein percent, respectively. Variability between herds for productive traits was lower than for SCC (6,459‑8,744 kg). The effect of all environmental factors considered was significant. This effect was less important in autumn. Factors related to hygiene during milking and bedding practices appear to be particularly important.