Studies were performed to determine the sensitivity and specificity of a new microtube method for the detection of coagulase production by Staphylococcus aureus. Rabbit plasma containing EDTA was added to and lyophilized in API microtubes. Two standard coagulase plasmas containing EDTA were used in the conventional macrotube test and served as a basis for comparison. No false-positive or false-negative reactions were encountered with the microtube system. With this system, 53% of the coagulase-positive strains tested were detected within 1 h after inoculation, 82% were detected after 2 h, 97% were detected after 3 h, and 99% were detected after 4 h. With the first conventional method, 45, 81, 96, and 98% of the positive strains were detected in 1, 2, 3, and 4 h, respectively, whereas with the second conventional method, only 6% of the positive strains were detected in 1 h, 24% in 2 h, 66% in 3 h, and 81% in 4 h. With the microtube method, 5 of the 139 coagulase-producing strains studied reverted to negative between 5 and 24 h after inoculation, whereas 9 reverted with the more rapid conventional method, and no reversions occurred with the second conventional method. All reversions involved strains which caused gelation of plasma within 1 h after inoculation. The data obtained showed that 99% of the coagulase-positive strains tested could be detected within 4 h by the microtube method. In addition, the microtube method offers a more convenient and economical format for the performance of the coagulase tube test.