Abstract: Two farms were examined for the presence of mastitis. The incidence of the disease was 20.1 per cent for the 1st farm and 36.4 per cent for the 2nd one. Chemical analysis for lactose and chloride contents and calculation of Koestler`s number revealed their alteration compared to normal cases. Generally speaking, the effectiveness of milk-borne organisms of the 2nd farm on milk lactose content was more potent than that of the 1st farm. The reverse effect was observed concerning the chloride content. The result proved that the decrease in milk lactose content and/or the increase in milk chloride content can be used as an index for suspecting the presence of subclinical mastitis.