The presence of phosphodiesterase IV has been demonstrated in the human fetal brain, liver and placenta as early as in the 6th week of intrauterine development. The enzyme activity in each tissue increases with gestation, being maximum at 18-21 wk and then decreases. The Km values of this enzyme for bis-(p-nitrophenyl)-phosphate hydrolysis in the brain, liver and placenta are 2.94 mM, 1.47 mM and 1.66 mM, respectively. Presence of sulfhydryl group in the active center of the placental enzyme has been demonstrated with the help of cationic study. EDTA inhibits the enzyme in all the three tissues. Effect of concanavalin A reveals the absence or unexposition of glucose, mannose and N-acetylglucosamine moieties in the active site of the enzyme in each of the three tissues. Maximum enzyme activity has been found to be localized in the soluble supernatant fraction obtained on centrifuging the brain and liver homogenate at 105,000 × g and in 20,000 × g pellet of the placenta. © 1982.