An investigation was made into microtitre complement fixation test (CFT) procedures suitable for the serological diagnosis of naturally occurring Brucella ovis infection in rams. A procedure similar to the Australian standard procedure for bovine brucellosis was unsatisfactory when applied to sheep. Modification of the procedure by use of an initial serum and anticomplementary control dilution of 1:8 and increasing complement fixation time to 60 minutes at 37 degrees C, greatly improved the efficiency of the test. A sensitivity of 100% was recorded for 59 serums from known infected rams and a specificity of 99.9% for 1593 serums from rams known or believed to be free of infection. Some aspects of applying CF tests to sheep serums are discussed.