A preliminary study was carried out to evaluate the effect of a staphylococcal mastitis vaccine on somatic cell count (SCC = number of somatic cells per milliliter of milk, an indirect indicator of mammary gland bacterial infection), and on milk production (kg of milk/day, known to decrease in animals with mastitis) under natural conditions in commercial farm. A total of 132 cows of the Navarra Holstein-Friesian breed were used. They belonged to four farms. A total of 982 milk samples, collected from these animals five months before and five months after the month of vaccination were involved in the study. Animals were boostered two months after the first vaccination. When compared with the control group, the vaccinated group had an increase of 17.8% (P < 0.05) in the proportion of animals which presented a lower RCS in the 5 months following vaccination than in the 5 months before vaccination. Also, vaccinated animals had in the 5 months following vaccination a lower RCS (20.6% reduction; P < 0.05) and a higher milk production (6.1% increment; P < 0.05) in relation to the control group, meaning that the economic benefit of the vaccine may be 13 times higher than the cost. The problems found when carrying out the study under the mentioned farm conditions were also analysed in this study.