Twenty-four-hour-old aggregates of human diploid skin fibroblasts are allowed to attach to a glass or plastic substratum. As a result of this attachment the cells in the aggregate demonstrate rapid and generalized changes in cell shape, cell surface and cytoplasm ultrastructure and in their ability to incorporate [3H]thymidine. Within 24 h they grow out on the substratum to attain the regular monolayer configuration. During the process of leaving the aggregate for the substratum a great number of different morphogenetic properties are displayed by the cells, resembling the properties of embryonic or epithelial cells. The simultaneous occurrence of this great variety of cell shape and cell surface changes, many of them unusual for fibroblasts, as well as the concurrent formation of organized cytoplasmic structures - microfilaments, microtubules - at localized areas of the cells, makes this system a potentially useful tool in the study of cell behaviour.