In Trial 1, 19 lots of Blastomyces dermatitidis (T-58; Tennessee dog isolate) were assayed to determine the stability of the reagents following storage. The reactivity of the antigens, produced from 1989 to 2012 and stored at 4°C, was determined by comparing antibody detection (enzyme-linked immunosorbent assay; ELISA) in 12 serum specimens from immunized rabbits. All of the 19 reagents produced during this 23-year period exhibited a high degree of stability and were able to detect antibody in the sera. Mean absorbance values ranged from 0.798 (1989) to 0.827 (2012) and a mean value for all 19 antigens of 0.728. In a related evaluation, Trial 2, B. dermatitidis lysate antigens prepared from 8 isolates (dog, human, soil) at two different time periods were assayed as above to determine reactivity. The time of storage between the first and second reagents varied from 4 to 17 years. The results indicated that all 16 of the lysate antigens detected antibody in the 15 rabbit serum specimens with mean absorbance values ranging from 0.346 to 0.682, but variations in reactivity were observed depending on the lysate and the serum specimen assayed. This comparative study provided evidence that the antigenic reagents do exhibit some lot-to-lot variation in reactivity, but they did not lose any appreciable potency during prolonged storage.