Human estrogenic 17beta-hydroxysteroid dehydrogenase (17beta-HSD1, EC1.1.1.62) is an important enzyme that catalyses the last step of active estrogen formation. 17Beta-HSD1 plays a key role in the proliferation of breast cancer cells. The three-dimensional structures of this enzyme and of the enzyme-estradiol complex have been solved (Zhu et al., 1993, J. Mol. Biol. 234:242; Ghosh et al., 1995, Structure 3:503; Azzi et al., 1996, Nature Struct. Biol. 3:665). The determination of the non-reactive ternary complex structure, which could mimic the transition state, constitutes a further critical step toward the rational design of inhibitors for this enzyme (Ghosh et al. 1995, Structure 3:503; Penning, 1996, Endocrine-Related Cancer, 3:41). To further study the transition state, two non-reactive ternary complexes, 17beta-HSD1-EM519-NADP+ and 17beta-HSD1-EM553-NADP+ were crystallized using combined methods of soaking and co-crystallization. Although they belong to the same C2 space group, they have different unit cells, with a = 155.59 A, b = 42.82 A, c = 121.15 A, beta = 128.5 degrees for 17beta-HSD1-EM519-NADP+, and a = 124.01 A, b = 45.16 A, c = 61.40 A, beta = 99.2 degrees for 17beta-HSD1-EM553-NADP+, respectively. Our preliminary results revealed that the inhibitors interact differently with the enzyme than do the natural substrates.