This survey was conducted for the serological confirmation on large outbreak of bovine brucellosis in two dairy farms. Serological tests were performed by the plate agglutination test, tube agglutination test, enzyme linked immunosorbent assay(ELISA), complement fixation, ,test(CFT) and rose bengal plate test(RBPT). Total 200 heads(134 heads in farm A and 66 heads in farm B) were tested. The primigravida and positive group have been raised separately in the farm A and both group have been raised together in the farm B.. The result were summarized as follows ; 1. Positive ratios in positive herds of farms by the tube agglutination test were 68.3% in farm A and 53.2% in farm B. 2. Seroconversion to brucella was observed in the primigravidas group in farm B, but was not observed in the primigravidas group in farm A. 3. All calves born in positive herd were serologically negative at time of test. 4. Positive ratio of ELISA in farm A was higher than that of tube agglutination test. 5. Number of positive reactors by the CFT, RBPT in farm A were equal to those of tube agglutination test.