The net rate of spontaneous aggregation of cells suspended with EDTA was measured for various cell types including spontaneous transformants and cells transformed with DNA and RNA viruses. The anchorage dependence as determined by growth in methyl cellulose and the tumorigenicity in vivo were also determined. All cells that had lost their anchorage dependency and were tumorigenic showed a high net rate of spontaneous adhesion. A31 was the only nontransformed cell line to have a high net rate of adhesion. The net rate of spontaneous aggregation of cells is a quick and reliable index of tumorigenicity and offers a new approach to understanding the mechanisms of cell surface changes associated with transformation.