This report describes the selection and behavior of tumor cells resistant to cytolysis by syngeneic lymphocytes. Two B16 melanoma lines, F1 (low metastasis) and F10 (high metastasis), were cultured with lymphocytes from C57BL/6 mice immunized against B16. The selection procedure involved repeated exposure of the tumor cells to lymphocytes in vitro. After each interaction, the viable tumor cells were trpsinized, replated, and designated lines F1Lr-1 and F10Lr-1. The procedure was repeated five times, yielding lines F1Lr-6 and F10Lr-6, which resisted cytolysis by syngeneic lymphocytes. Mice were given s.c. or i.v. injections of cells from lines F1, F1Lr-6, F10, or F10Lr-6. Tumor growth patterns were the same for all four lines when the cells were injected s.c., however, the incidence of pulmonary metastases differed significantly after i.v. injection. Line F10 cells yielded more pulmonary metastases than an equal number of line F1 cells (p less than 0.01). F1Lr-6 cells yielded significantly fewer metastases than an equal number of lines F1 cells (p less than 0.01). A similar difference between F10Lr-6 and F10 cells was observed. The incidence of artificial metastases after i.v. injection of F10Lr-6 cells was similar to that for F1 cells. The quantitative organ distribution, arrest, and survival of i.v.-injected tumor cells were studied by using [125]-5-iodo-2'-deoxyuridinelabeled cells. There was a significantly greater number of cells from line F10, arrested and able to survive for 14 days in lungs, than cells from line F1. In contrast, cells from either line F1Lr-6 or F10Lr-6 had a lower incidence of arrest and survival than their lymphocyte-sensitive counterparts.