The Lewis lung carcinoma is a transplantable metastatic tumor of the C57BL mouse strain. When cells derived from this tumor are inoculated onto the chorioallantoic membrane (CAM) of 9- to 11-day-old chick embryos, large tumors are produced. Although most of these tumors contain mouse cells, they can no longer be transplanted either in C57BL mice or on the CAM. Cloned cells obtained from Lewis lung carcinoma grown in vitro produce CAM tumors which are different from those produced by the parental cells in that the former retain some tumorigenic potential in mice. The phenotype associated with the cloned cell populations appears to be stable in vitro and in vivo. Phenotypes similar to those observed when both the parental and the clonal populations of Lewis lung carcinoma cells are grown on the CAM can be reproduced in culture conditions.