Halichondrin B and homohalichondrin B are novel tubulin-interacting agents isolated from marine sponges. The in vivo antitumor activities of these compounds were examined in human tumor models in immunodeficient mice and rats. In nude mice, regression or pronounced delay of subcutaneous tumor growth was obtained with both halichondrins, at a maximum tolerable dose of 20 micrograms/kg Q2Dx5, in three of four vinblastine-sensitive tumors, including two melanomas and one osteosarcoma; one small-cell lung cancer line was resistant. The halichondrins as well as vinblastine showed only marginal activity against KM20L colon carcinoma xenografts. In a LOX melanoma lung colony formation assay in groups of six nude mice, all control animals were sacrificed because of respiratory symptoms 38 days after cell injection, and likewise one vinblastine-treated mouse after 53 days. All halichondrin-treated mice in the lung colony assay appeared healthy throughout an observation period of 112 days (p = 0.002). Upon necropsy all vinblastine-treated animals, and two of six mice in the halichondrin group, had macroscopic lung tumor colonies. In a nude rat model for LOX bone marrow metastases, the mean lifespan of untreated control animals was 15 days. Whereas vinblastine had only a marginal effect (17 days) in this model, halichondrin B prolonged the lifespan of the animals to 32 days, representing a significant (p = 0.0016) difference between the two compounds. In conclusion, the halichondrins, which comprise a subtype of tubulin-interactive anti-mitotic agents, showed distinct antitumor activity profiles in human tumor models, thereby encouraging their further preclinical development and possible clinical evaluation.