The viability of the placental villi in organ culture was investigated both from the morphological and functional points of view. Morphological viability was judged on the appearance of the villi with special emphasis on the trophoblastic cells and the morphological integrity of the placental villi could be maintained for at least 96 h in organ culture. Functional viability was judged by analyses of delta 5-3 beta-hydroxysteroid dehydrogenase; delta 5,4-isomerase (3 beta-HSD) activity with 14C-pregnenolone as the substrate and by the suppressive effect of the cultivated villi on the secretion of plasminogen activator (PA) obtained from mouse peritoneal macrophages stimulated by OK-432. It was found that the 3 beta-HSD activity and suppressive effect on PA secretion could be preserved for at least 120 h in organ culture.