The antibody titres of 149 cows aborting leptospire infected fetuses and 156 cows aborting fetuses not apparently infected with leptospires were determined by the microscopic agglutination (MA) test. There were differences in the distribution patterns of MA titres between the two groups of cows but these differences would only be of very limited value in the diagnosis of leptospiral abortion. Although 81.2 per cent of aborting cows which had a titre of 1:1000 or greater had leptospire infected fetuses, they represented 46.3 per cent of cows aborting leptospire infected fetuses. Among cows which aborted infected fetuses, 22.8 per cent had no detectable antibodies. The complement fixation and plate agglutination tests were of less value than the MA test.