The growth of colonies of human glioma cells cultured in agarose has been studied. The mitotic activity and the fraction of cells which incorporated tritiated thymidine decreased nearly exponentially with the depth in the colonies, the shape and inclination of the proliferation gradient being independent of the colony diameter. This explains why the volume doubling times were rather similar for colonies of different sizes. It is also shown that the stability of the gradient during growth gave rise to nearly exponential growth, although a large part of the centrally situated cells were out of the cell cycle or had a reduced proliferative rate. The incorporation of tritiated thymidine into labelled cells was similar in the peripheral and the central regions as evaluated from grain counting. This indicated that the length of the S-phase was constant.