Cytoenzymological reactions for SDH, LDH, Iso-CDH, G-6-PDH and cytochrome oxidase activity were tested on unfixed cultures of chick-embryo fibroblasts. The cultures were not passaged and the basic medium was unchanged throughout the experiment. The authors found that the enzymatic activity in the cells changes and increases with ageing of the culture within the fourth and sixth day. On the sixth day the activity disappears because of regressive changes in the cells. The observed individual differences in intensity of the enzymatic reactions in individual cells of the same kind and within the same culture result from various metabolic states of these cells.