Comparative tests were carried out on serum samples using the Rose Bengal test (RBT), the complement fixation test (CFT), the antibovine globulin test (ABGT) by tube, plate and rapid variants, the mercaptoethanol test (MET) and the dithiothreitol test (DTT). Forty cows, from which Br. abortus had been recovered, gave strong reactions in all tests except for 2 cows in the SAT and 3 cows in the DTT Another group of 405 cows had not yielded Br. abortus on limited bacteriological examination. There was good correlation between the results of the CFT, RBT, tube ABGT, rapid ABGT and, to a lesser extent, the MET in cows with a CF titre greater than or equal to 16 or less than 4. In cows with CF titres of 8 the correlation was not as good while cows with a CF titre of 4 gave a wide range of reactions in these supplementary tests. A plate variant of the ABGT gave titres which were generally lower, by about 1 dilution, than those in the tube procedure. The results of the rapid ABGT correlated well with the results of the tube ABGT and the CFT in samples of high and low titre but not as well in samples of intermediate titre. The DTT gave titres which were usually lower than the corresponding MET titre. On the basis of the results obtained, tentative standards for the interpretation of the ABGT (tube) and MET were defined. It was concluded that these tests could be used to elucidate the status of a proportion of the cows which gave inconclusive results in the CFT.