Activity and a few physiochemical properties of acid phosphatase (ACP) have been studied in different subcellular fractions of developing human placenta and fetal liver. The enzyme activity is discernible in all subfractions of each tissue already in the 6th week of gestation. The activity of the hydrolase in 20,000 g pellet. 105,000 g pellet and in 105,000 g supernatant of the placenta and in all fractions of the liver increases significantly with advancement of pregnancy until 26 weeks and declines in term placenta. However, 800 and 10,000 g pellets of the placenta show a gradual decrease in ACP activity with progression of gestation. Experiment with different inhibitors reveals that each tissue contains at least two types of ACP - a soluble one and a sedimentable one. It appears from the Lineweaver-Burk plot of kinetic study that sedimentable form has low Km and high Vmax values. Ag+, Cu++, Hg++, Co++ and Zn++ are inhibitory, while Fe++ and EDTA are stimulatory to the enzyme. The effect of different detergents, particularly with sodium deoxycholate and Triton X-100, shows the increase of the hydrolase activity, indicating ‘latent nature’ of the enzyme. © 1982 S. Karger AG, Basel.