Two human choriocarcinoma cell lines, BeWo and Jar, were used as a model system to study 17β-hydroxysteroid dehydrogenase (17β-HSD) activity. Northern blot, Western blot and 3H-water assay were performed to investigate the mRNA species, protein and enzyme activity of 17β-HSD. Two sizes of 17β-HSD mRNA, 1.3 and 2.2 kb, were detected in BeWo and Jar cells. In BeWo cells the predominant mRNA species is 1.3 kb. Western blot analysis demonstrated high level of 17β-HSD immunoreactivity and the 3H-water assay demonstrated significant enzyme activity in BeWo cells. In contrast, in Jar cells, the predominant mRNA species is 2.2 kb. Jar cells contain very low 17β-HSD enzyme activity and Western blot failed to show detectable 17β-HSD immunoreactivity. It is possible that the 2.2-kb 17β-HSD mRNA cannot be efficiently translated into protein because of the presence of multiple AUGs and termination signals (UGAs, UAAs, and UAGs) at the 5’-untranslated region of the 2.2-kb mRNA.