The kinetics of initial arrest in organs, distribution, survival, and fate of 125I-iododeoxyuridine-labeled B16 melanoma tumor cells injected intravenously into normal, tumor-sensitized and immune manipulated syngeneic and allogeneic mice were investigated. Groups of animals were killed at intervals ranging from two minutes to 14 days after intravenous tumor cell injection. Lungs, liver, spleen and blood were collected from each animal and processed so that radioactivity associated with DNA of tumor cells viable at the time of sacrifice could be monitored. The following conclusions can be made: initial tumor cell arrest in organs is influenced by the host immune status but it does not correlate with the survival kinetics or development into tumors. The same tumor, which was rejected in mice after a subcutaneous tumor challenge, grew in the lungs after intravenous injection. Therefore, rejection of a subcutaneous challenge as the sole criterion of host immunity to neoplasms should be questioned. Allogeneic animals are not appropriate for use as a model system for the study of experimental metastasis. Animals sensitized to a tumor exhibit kinetic patterns of tumor cell arrest and survival that differ from normal syngeneic hosts.