The capacity of cells to form growing tumor-like colonies in culture was tested by a new and unlaborious suspension technique. Six of 9 tested cell lines formed spheroids, 5 of which started to grow. The growing spheroids reached a maximal size determined by balanced cell gain in a proliferative, superficial layer and cell death, particularly at larger depth. Sections of spheroids of different origin showed large variations in the thickness of the viable cell layers and in the shape of the proliferative gradients. The data, taken together with earlier published information, indicate that a difference between rodent and human cells exists, the former showing thinner viable and proliferative layers.