BSp73 arose spontaneously (1979) as intraperitoneal nodules together with ascites. Histologically, the nodules were classified as adenocarcinoma of the pancreas. During serial transplantation of ascites cells to a subcutaneous site, two variants appeared--one fast-growing, nonmetastasizing, the other slowly growing and metastasizing via the lymphatic system to the lung. From the in vivo selected variants as well as from the parental tumor, two types of tissue cultured cell lines were established. These differed in morphology, adherence to plastic, susceptibility to detachment by trypsin, and, above all, in the ability to metastasize upon reinjection into syngeneic recipients. From differences in growth kinetics in vivo it has to be concluded that variant cell types coexisted in the parental tumor in the form of precursors, which adopted their characteristic features (irreversibly) after a shift in the environment.