Subclinical mastitis (SCM) is reduction in quality and quantity of milk with no visible changes in udder and it is more critical than clinical mastitis. It has multiple etiology, which include nutritional deficiency, number of lactations and unhygienic managemental practices, which introduces infectious agent in to the udder. Diagnosis of subclinical mastitis is more problematic than mastitis and can be made in a variety of ways including direct measurement of the somatic cell count (SCC) level or indirectly by performing a California Mastitis Test (CMT) on suspected quarters. The use of intramammary antibiotics to treat cows sub-clinically infected with Streptococcus agalactiae is usually successful and results in increased production and dramatic decreases in bulk tank SCC. Treatment of subclinical mastitis with penethamate is effective and sustain positive effect to reduce SCC in milk.