THE rapid rate of cell division in early embryogenesis is accompanied by rapid synthesis of DNA1. The function of deoxyribonuclease II (DNase) is essentially catabolic and is probably concerned with the breakdown of DNA from dead cells. DNase would then be expected to be required by the body shortly after embryogenesis and its time of appearance in the star-fish, frog and chicken is essentially similar2. During early embryogenesis of the chick DNase II is difficult to detect but increases rapidly after four daysof incubation2. The onset of DNase activity in the whole mouse embryo and the developing mouse liver has been investigated and been shown to follow a similar ontogenetic pattern as in the other species named here.