Aim - The aim of the present paper was to compare some screening tests, such as California Mastitis Test (CMT), pH, somatic cell count (SCC), electrical conductivity (EC), and bacteriology for diagnosis of mastitis in dairy cattle. The second aim of this work was to evaluate the predictive index of CMT, pH, SCC, EC and bacteriological exam for the diagnosis of mastitis in dairy cattle. Materials and methods - Fifty-four Frisian cows of different age were included in the present study. Udder was evaluated clinically in all the animals. Milk samples were collected during the evening milking. Electrical conductivity has been evaluated directly in the milking room by the milking machine, along with California Mastitis Test (CMT). Then milk samples have been collected in sterile tubes from each quarter. Samples have been refrigerated at 4°C and evaluated within one hour for bacteriology and milk pH. Milk samples for SCC have been collected in PVC bottles added with a preservative preservative and specimens have been processed in the Breeder County Association laboratory (Associazione Provinciale Allevatori - APA). Results - No dairy cattle included in the present study showed clinical signs of mastitis. Clinical score was compatible with healthy animals or with animals affected by sub-clinical mastitis. Our results showed a CMT positive in 28/54 cattle. Electrical conductivity was higher than 11 Ohm in 19/54 animals and somatic cell count (SCC) was higher than >200.000/ml in 19/54 animals; pH was changed in 4/54 dairy cattle. The bacteriological exam was positive in 10/54 dairy cattle. Bacteria isolated were predominantly environmental pathogens. Statistical analysis showed a positive predictive index for SCC and bacteriology. Conclusions - Statistical analysis showed a high probability to have a positive bacteriological culture if SCC was higher than 200.000 cell/ml. These results support the accuracy of SCC as a screening test for the diagnosis of sub-clinical mastitis and for clinical mastitis in association with a modified milk secretion and/or a modified udder. CMT also has been proved as a good screening test to be used during milking in all the animals with suspect mastitis.