Abstract 1. 1. The neuraminidase (N-acetylneuraminate glycohydrolase, EC 3.2.1.18) activity of subcellular fractions obtained from chorioallantoic membrane of chick embryos was determined. More than 90% of the activity was found in a supernatant fraction. 2. 2. Neuraminidase activity was determined in 13 tissues of the 11- and 18-day-old chick embryo and in 11 tissues of the 7-day-old chick. Those tissues which are derived partially or completely from the ectodermal layer showed a 10–20-fold (chorioallantoic and amniotic membranes, skin and cloaca) or a 2–3-fold (brain, eye, spinal cord) increase in activity as the embryo grew older. In each case the activity decreased after hatching. The other tissues showed low activity at all stages. Similar results were obtained with tissues from 14- and 26-day-old duck embryos, though the increases in activity were not as great as with the chick embryo. 3. 3. There was no apparent connection between the substrate concentration (total sialic acid) and neuraminidase activity in any organ. 4. 4. These changes in neuraminidase activity were not found in tissues from mouse or guinea-pig embryos. The increase of enzyme activity in some tissues of the chick embryo may be due to the accumulation of a humoral factor in the egg.