The activity of nonspecific phosphomonoesterases in cerebellar structure of newborns and infants with trisomies in the 13th (5 cases) 18th (4) and 21st (14) chromosomes was investigated. In all trisomies, changes of phosphatase activity in the cerebellar structures were demonstrated which were particularly marked with alkaline phosphatase. The activity of this enzyme increased not only in the vascular endothelium but in a number of cellular structures as well: in the external embryonal and internal granular layers of the cortex, in the nuclei of Purkinje cells and neurons of the nucleus dentatus, in the foci of heterotopia of cortical cells into the white matter and nucleus dentatus. In Edwards' syndrome the activity of alkaline phosphatase in cellular elements of the cerebellum was slightly higher than in the control group. Changes in the activity was observed in the cells of the external embryonal layer, neurons of the nucleus dentatus and in the cells of the heterotopic foci. In autosomal trisomies dystopic cells have dissimilar levels of phosphatase activity and stain mosaically which seems to confirm the assumption of their dissimilar differentiation.