Human estrogenic 17β-hydroxysteroid dehydrogenase (17β-HSD1, EC1.1.1.62) is an important enzyme that catalyses the last step of active estrogen formation. 17β-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, 17β-HSD1-EM519-NADP + and 17β-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 9, b=42.82 9, c=121.15 9, β=128.5 o for 17β-HSD1-EM519-NADP + , and a=124.01 9, b=45.16 9, c=61.40 9, β=99.2 o for 17β-HSD1-EM553-NADP + , respectively. Our preliminary results revealed that the inhibitors interact differently with the enzyme than do the natural substrates.