Abstract: Two different farms were subjected to an investigation for the presence of mastitis. The incidence of the disease was 20.1 and 36.4 percent in the 1st and 2nd farm respectively. Bacteriological analysis revealed that in the 1st farm Staphylococcus spp and Streptococcus spp were the causative agents of the diseases, these cultures were isolated in a single form. In the 2nd farm, the causative agent was either a single culture of Staphylococcus spp or a mixed cultures of Staphylococci, Streptococci and Escherichia coll. The fat content of milk decreased greatly due to the infection. In the 1st farm, the decrease was greater when the infection was due to Staphylococci than Streptococci, while in the 2nd farm, the results varied greatly, but the overall decrease of the milk fat content of the 2nd farm was significantly greater than that of the 1st farm.