From C57BL mouse melanoma B-16 cells, variant clones were selected in vitro which were resistant to the lectins wheat-germ agglutinin and ricin. Cells were also selected which survived toxic concentrations of concanavalin A. Four different In vivo assays using intradermal, intravenous, intraperitoneal and intramuscular injections were used to assess the tumorigenidty and metastasizing capacity of these lectin-resistant variants. It was concluded that to obtain a complete picture of the malignant properties of a given cell line or clone, all four assays have to be carried out. In comparison with the parental cells, the WGA-resistant cells showed a most dramatic decrease in metastasizing capacity through both lymphatic and vascular channels. Tumorigenidty was also reduced. The ricin-resistant cells showed a defective development into lung tumors and thus displayed a reduction in metastasis through the hematogenous route. Since this line did not change its capacity to metastasize via the lymphatic route, and the tumorigenidty was not significantly altered, it will be a good model for studies seeking to dissociate these two properties. The Con-A-selected cells, when injected intravenously, developed tumor nodules in the liver in addition to those in the kings, while no striking alterations in tumorigenicity or metastasizing capacity could be detected in this line.