A total of 2,024 quarters of 515 dairy cattle in Kangweon area were examined for incidence of subclinical mastitis. Milk samples from cattle infected with subclinical mastitis were studied bacteriologically and the bacterial strains isolated were further examined for sensitivity to 12 antibacterial agents. And the status of carrying out the mastitis control program in 28 dairy farms was examined. The results obtained were summerized as follows ; 1. A total of 308(59.8%) of 515 cattle and 656(32.4%) of 2,024 quarters were found to be infected with subclinical mastitis. 2. The 277 strains of etiological agents were isolated from 358 subclinical quarters. These were identified as Staphylococcus aureus(14.4%), other staphylococci(36.5%), Streptoccus agalatiae(8.7%), other streptococci(30.7%), Bacillus spp.(1.8%), Corynebacterium spp.(1.4%) and coliform(0.7%). 3. The 109 strains of streptococci and 141 strains of staphylococci were examined for sensitivity to 12 antibacterial agents. All the strains of streptococci were sensitive to penicillin, ampicillin and cephalothin, and they were also sensitive to erythromycin(88.1%), clindamycin(83.5%), enrofloxacin(75.2%), trimethoprim＋sulfamethoxazole(67.9%), The strains of staphylococci were sensitive to cephalothin(97.2%), gentamicin(83.0%). enroflozacin(80.9%), trimethoprim＋sulfamethoxazole(78.0%), erythromycin(71.6%) and clindamycin(71.6%). But all the strains resisted to colistin. 4. In the 28 dairy farms examined, condition of udder before washing was dirty in most of the farms (89.7%). Hygiene of milking equipment was only good in the 5 farms(17.9%). Teat preparation before milking was good in the 6 farms(21.4%). The farms in which teat dipping after milking was conducted were 46.4%. Dry cow treatment for the complete herd was carried out in most of the farms(89.3%) but mastitis checking was only carried out in the 8 farms(28.6%) irregularly.