In vivo and in vitro investigations of a Syrian hamster tumor metastasis in the lymph node (H10Zm) in comparison with the primary tumor (H10Z) are performed. It is shown that the metastasis differs from the primary tumor by the prevalence of M secondary cell type characterized by globulous appearance, decreased adhesion and the presence of two long submetacentric marker chromosomes. Since a cell type with such cytogenetical characters was also found in the primary tumor as well as in a non-metastasizing tumor variant, it is suggested that the role of the induction-selection phenomenon in the occurrence of H10Z tumor metastases, i.e., new inductor agents occurred in the tumor's environment, might produce the induction of certain repressed mutant genes in M secondary cell type, bringing about certain phenotypical alterations (e.g., the decrease of the mutual cell adhesion) which could favorize the migration and selection of this cell type.