The apoenzyme of the human placental 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) and its complex with NADP+ were prepared from two alternative procedures. The apoenzyme (Form I) has an absorption maximum at about 279 nm, and an absorption ratio at 280 and 260 nm of 1.65 +/- 0.1; whereas the complex (Form II) has a broad absorption peak between 268-278 nm, and a 280 to 260 nm ratio of 1.1 +/- 0.05. Upon addition of the substrate estradiol to the complex, an absorption increase at 340 nm and a fluorescence emission at 450 nm, following NADPH formation, were produced. Both changes indicate that one cofactor is tightly bound to the 17 beta-HSD molecule in this complex. No significant optical change can be produced in this way for the apoenzyme. Convenient analyses of cofactor content of the enzyme are thus provided. The optical analyses and the homogeneous apo- or holo-enzyme preparations are important in the study of the enzyme's function and crystallization. This is the first human steroid converting enzyme which has yielded X-ray quality crystals.